REPRODUCTION 73 



within the anterior third of its length. Except for the dorsal flattening, 

 the configuration of layers is as simple as possible. Figures 46B-O show 

 transverse sections at stages successively later than that of Fig. 46 A. 

 Several things are happening simultaneously. A broad band of dorsal 

 ectoderm (NP), slightly thicker than the adjacent regions of the layer, 

 becomes separated, along its right and left edges, from the neighboring 

 ectoderm. This process involves the mid-dorsal ectoderm continuously 

 from the blastopore almost to the anterior end of the embryo. The 

 median ectoderm thus delimited from the lateral ectoderm is the material 

 of the prospective central nervous organ, the neural tube. In this initial 

 stage it is called the neural (or medullary) plate. 



The dorsal endoderm is at first flattened in comformity with the neural 

 ectoderm but later (Fig. 46, D-F) it becomes convoluted along three fines 

 extending lengthwise of the embryo. Its median slightly thicker region 

 becomes sharply folded upward. Either side of this median fold a longi- 

 tudinal groove appears on the inner surface of the endoderm. Then the 

 endoderm in the region of each of these grooves assumes the form of a 

 fold extending outward dorso-laterally. Thus arise three folds, one 

 median and a lateral pair, all convex outward, and extending nearly the 

 whole length of the embryo. As time goes on these folds become more 

 emphasized, but soon a difference arises between the median fold and the 

 lateral folds. The median fold remains continuous throughout its entire 

 length. The lateral folds, however, become interrupted by the formation 

 of sharp deep transverse folds which cut from above downward through 

 each lateral fold. This process of subdivision or segmentation begins 

 near the anterior ends of the lateral folds. Its immediate result is a pair 

 of approximately globular pouches lying symmetrically either side of the 

 median fold, each pouch having a small central cavity opening by a narrow 

 passage into the archenteron. Later this passage is closed and then the 

 pouch becomes detached (Fig. 46F) from the archenteric wall which, at 

 the place where the pouch had formed, closes so that nothing is left to 

 mark the spot (Fig. 46G). 



Immediately behind each pouch of the first pair another similar pouch 

 forms exactly as the first did. At this stage of development, marked by 

 the presence of two pairs of these pouches, the embryo escapes from the 

 egg membrane ("hatches"). The period between fertilization and hatch- 

 ing varies considerably, its average being probably not far from twelve 

 hours. 



These two pairs of pouches derived from the dorso-lateral endodermal 

 wall of the archenteron constitute the first definitely delimited mesodermal 

 material. The remainder of the dorso-lateral folds, extending back to 

 the blastoporal region, is destined to give rise, after hatching, to additional 

 mesodermal pouches. The median endodermal fold, which has remained 



