214 EDWIN G. COXKLIN. 



anterior pair of cells of the yellow, then in the pair of yellow, 

 cells adjoining the latter behind, and finally in the posterior 

 median pair. The subdivision of the cells of the gray crescent 

 occurs at the 44-cell stage ; the ventral product in each case is 

 a small clear cell which ultimately forms the posterior portion of 

 the neural plate, the dorsal products are larger and are gray in 

 color and ultimately develop into the chorda. 



In the case of the yellow crescent both products of the pos- 

 terior median cells give rise to mesenchyme, but the ventral ones 

 contain those caps of clear protoplasm which were visible in the 

 8-cell stage (Photo 1 1) and which in a still earlier stage first ap- 

 peared around the entering spermatozoon (Photo 3). The dorsal 

 products of the other two pairs of crescent cells give rise to mesen- 

 chyme, while from the ventral halves come the muscle cells of 

 the tadpole's tail. The mesenchyme cells are clear and faintly 

 yellow in color, the muscle cells are a deep yellow and these two 

 substances are distinguishable in their definitive positions even as 

 early as the i-cell stage (Photo 3). 



Unfortunately no photographs of the early stages of gastrula- 

 tion were taken. I have however studied and drawn every step 

 of this process both in living and in prepared material. Gastrti- 

 lation begins with the depression of the endoderm cells just 

 posterior to the chorda cells, and is later continued by the roll- 

 ing in of cells around the margin of the blastopore. In this 

 manner the neural plate cells come to overlie the chorda cells, 

 and the muscle cells, the mesenchyme. 



In the closure of the blastopore the posterior (ventral) lip 

 remains stationary until the last stages of the process, while the 

 anterior (dorsal) lip grows backward over the gastrocoel until 

 the blastopore is reduced to a longitudinal groove between the 

 muscle cells of each side (Photo 17). In the overgrowth of the 

 dorsal lip the rows of muscle cells as well as the blastopore groove 

 are forced to the hinder end of the embryo and the muscle rows 

 are tilted up at their anterior ends until they are transverse to 

 the long axis (Photo 17). Later the ventral lip overgrows the 

 remnant of the blastopore and the ectoderm of this lip forms a 

 pair of V-shaped folds which fuse from behind forward and thus 

 cover the dorsal lip and roll the neural plate up into a tube. In 



