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The various aspects of chemical crosslinking are addressed. Crosslinker reactivity, specificity, spacer arm length and solubility characteristics are detailed. Considerations for choosing one of these crosslinkers for a particular application are given as well as reaction conditions and practical tips for use of each category of crosslinkers.

SIXTEEN PLATES (TWO HUNDRED FIGURES) AUTHOE'S ABSTMCTAlthough the egg of Amphioxus is much more fluid and less stereotyped than that of Ascidians. the poles, axes, and localizations of formative materiala are much the same in the two. The spermatozoon enters near the vegetntive pole and a peripheral layer of granular cytoplasm flows to this pole and later forms a crescent around the posterior side parallel to the first cleavage amphiaster; this is the mesodermal crescent, On the anterior side a similar area later gives rise t o the chorda-neural crescent. Above these crescents is the ectodermal area, below them the endodermal area. The early cleavages divide these crescents and areas just as in Ascidians The coelohlastula is at first spherical. but later flattens i n the region of the mesodermal crescent; this flattening extends forward on the vegetative side to the chorda-neural crescent, where the invagination is sharpest.The blastopore is a t first triangular i n outline, the dorsal lip being formed by the chorda-neural crescent and the lateral lips by the mesodermal crescent. Later the chorda and mesodermal crescents are infolded, the lateral lips fuse to form the ventral lip which grows dorsalward the blastopore becomes wider from right to left than dorsoventrally, the gastrula elongate; and i n the angles between dorsal and ventral lips the mesodermal crescent forms the mesodermal grooves in the lateral walls of the castrocoel.

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