PREFACE which a knowledge of mammalian embryology is the objective, ordinarily pass from the study of three or four day chicks to work on mammalian embryos. While the text has been kept brief, illustrations have been freely used in the belief that they convey ideas more readily and more accurately than can be done in writing. Direct labeling has been used in the figures to facilitate reference to them. Most of the drawings were made directly from preparations in the laboratory of Histology and Embryology of Western Reserve University School of Medicine. However, figures from other authors, particularly LilHe and Duval, have been used extensively for comparisons and for schemes of presentation. Several figures have been reproduced directly or with only slight modifications. These are designated in the figure legends. I wish to acknowledge the assistance I received in the preparation of material by Mrs. Mary V. Bayes, and in the drawing of the figures by Mrs. Bayes and Dr. Louis J. Karnosh. I am "also indebted to my father, Prof. Wm. Patten of Dartmouth College for criticism of the figures, and to Dr. F. C. Waite of the School of Medicine, Western Reserve University for his helpful interest and cooperation in all phases of the preparation of the book and especially for his reading of the manuscript.

Resumen por el autor, R. M. Patten.Ida forniacih del asa cardiaca del pollo.Las fases tennpranas de! estahlecimiento del corazhn del pollo J' 10s estados ulteriores de divisi6n en diferentes chmaras hari sido cuidadosarnente investigados por diversos autorw. E l presente trabajo se ocupa de 10s procesos interniedios algo familiares, pero hasta el presente nienos completamente descrifos, de la formacihn del asa y la diferenciacih regional temprana, bashdose en disecciones, reproducidas en niodelos plhsticos, j* en reconstrucciones en cera. Se ocupa de 10s siguientes puntos: 1. La forniacihn en el tubo cardiaco de un asa en foriiia de U dirigida hacia cl lado derecho, y dc algunos de 10s factores causativos inrwcados en cstt proceso. 2. La formacibn del asa cardiaca y la relacihn de la torsi6n y flexihn del cuerpo del enibrihn con el pi*oceso de la forniacihn de dicha asa. 3. La difrrenciacihn regional (!el coraz6n cn el bulbo cardiaco, ventriculo, atrio y seno renoso, y loi canibios tenipranos cn cada una de estas regioncs.

NOLD, 2 in 1894, reported on a series of hydrocephalic individuals exhibiting, also, congenital defects in the region of the hindbrain. Although these cases varied considerably, the essential condition was that of a herniation of tongue-like processes of cerebellar tissue into the cervical canal. In 1891 and 1896 Chiari 5,6 described a series of similar cases in which a distorted medulla, as well as cerebellar tissue, was within the cervical canal. In these cases the tongues of cerebellar tissue were so bound down to the medulla that usually there was interference with the passage of cerebrospinal fluid through the roof of the 4th ventricle. Schwalbe and Gredig 21 suggested that such cases formed a logical entity which they called the Arnold-Chiari malformation.The frequent association of an Arnold-Chiari malformation with myeloschisis has led certain workers to postulate a causal interrelation between the two conditions (see, for example, Penfield and Coburn, Is and Lichten-steinl2). It is well known that, during the middle trimester of gestation, the spinal column increases in length more rapidly than the spinal cord. 11,~6,22 Normally at this time the cephalic part of the cord is more securely anchored than its caudal region. This is not only because of its fixation by the brain within the developing cranium, but also because of the relatively more advanced state of development of the spinal nerves cephalically. As a result of the differential in rate of growth and the cephalic anchorage, the lumbosacral part of the cord slides cephalad within the neural canal with the concomitant elongation of the lumbosacral nerve roots to form the cauda equina. If during this period there is a lumbar or a sacral myeloschisis the spinal cord has an abnormal point of fixation caudally. This undoubtedly subjects it to abnormal traction, and the inference that such traction might pull the medulla and cerebellum caudad through the foramcn magnum, is plausible and has all the allure of a "simple mechanical explanation" of the genesis of the Arnold-Chiari malformation. The validity of the traction interpretation has been called in question by a number of workers. 1,3,13-15 MATERIAL There are available in the University of Michigan Embryological Collection five human embryos showing various stages in the development of * The participation of Dr. Stewart in this work was made possible through a summer research fellowship granted to him while he was a medical student. ~85

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