The normal pattern of renal development late in gestation in rats involves a transient variation in growth rates. While the renal papilla is slowly and steadily increasing in length the renal parenchyma is rapidly and almost exponentially increasing in weight. This disparity in growth rates frequently results in a kidney that has an enlarged renal pelvis late in gestation; but this is only an “apparent hydronephrosis” since it disappears shortly after birth. Methyl salicylate retarded renal development, particularly growth of the renal papilla, and increased the number of both transient and permanent renal abnormalities in rat fetuses near term. Preliminary evidence suggested that if the renal papilla has reached a particular stage of development 1 day before birth, and is not accompanied by grossly apparent renal malformations, the kidney will be normal at weaning. However, at birth, kidneys with an absent renal papilla, but that appeared otherwise normal, cannot be differentiated on a gross anatomical level from those that are destined to be permanently abnormal.

The value of the guinea pig for investigations of teratogenesis and pregnancy is enhanced by its relatively long gestation. This species-characteristic provides especial opportunity to study deleterious factors which act differentially with respect to the phases of pregnancy as do many noxious factors in man (Taussig, '36). Not until the present, however, have sufficient data been available for a statistical analysis of the temporal aspects of pregnancy, both normal and abnormal. The uncertainty which has existed with respect to this aspect of gestation is reflected in the literature on the length of gestation in this species. Kenneth ('47) lists nine estimates ranging from 63 to 70 days. Inasmuch as the length of gestation varies inversely with litter size (Minot, 1891; McKeown and Macmahon, '56), it is possible that some of the differences are explained by the failure of investigators to make allowance for this relationship. It seems more likely though that the shorter estimates resulted from the inclusion of a considerable propor-'This investigation waa supported in part by research grant M-504

A brief description of the basic patterns of mammalian development of the eye is presented based on events as they occur in human beings. The emphasis is not on the details of this development, but rather on its organization and timing, with a figure of comparative development providing a comparison of similar events in man, rat, mouse, and chick.To an individual struggling to understand some basic mechanisms associated with normal embryology, the development of the eye is well worth study. Here induction has been clearly established as an essential mechanism, and one can discover examples of both primary and secondary inductive forces. Here, too, one finds examples of early developmental "steps" or events whose persistence (or absence) results in either congenital abnormalities or future "disease states." In this system one encounters examples of those unexplained "spontaneous events" which are so common in an embryologist's lexicon. An examination of the development of the eye will serve to remind us that embryogenesis is a continuum. Although discursive presentations such as this tend to emphasize the pieces rather than the puzzle, one should not lose sight of the whole when discussing its parts, for each event we shall discuss is part of an interdependent continuum which can be totally disrupted by the failure of any one of its components either to appear on time or to develop normally. Finally, the development of the eye serves to remind us that there are many instances in normal embryology in which development continues after birth and can be significantly altered by events in postnatal life. The presentation which follows is intended only as an outline of the development of the human eye. For those who wish to pursue to subject, a list of references has been supplied, and a figure of comparative development has been provided to remind you how rapidly these events occur in rodents as compared to man. April 1982Now let us turn out attention specifically to the development of the eye. We see the optic primordia first in man on about day 22 as bilateral evaginations of the neuroectoderm of the forebrain (prosencephalon) which still remains open as bilateral neural folds. These evaginations, the optic peduncles, continue to proliferate laterally as the forebrain closes so that at about day 27 they have become large, single-layered vesicles, the primary optic vesicles, which are continuous with the third ventricle. As they reach the surface ectoderm, these hollow balls of neuroectoderm, connected to the brain through hollow optic stalks, induce the formation of lens primordia, the lens placodes. This is the classical example generally offered as evidence for primary induction in normal development for, unless the optic vesicle reaches the surface ectoderm, no lens vesicle will develop. A groove appears on the inferior surface of each optic stalk and optic vesicle at about 29 days. This retinal (choroid or optic) fissure incorporates both mesenchyme and the hyaloid artery and vein by 33 days, vascularizing the ...

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A brief description of the basic patterns of mammalian organogenesis of the gastrointestinal and urogenital systems is presented based on events as they occur in human beings. The emphasis is not on the details of this development, but rather on its organization and timing, with tables of comparative development providing a comparison of similar events in man, rat, mouse, and chick.