52 



CHORDATE ANATOMY 



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

 become more emphasized, but soon a difiference arises between the median 

 fold and the lateral folds. The median fold remains continuous through- 

 out 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 sym- 

 metrically 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. 42F) 



^MES 1-6^ EC 



Fig. 43. — AMPHIOXUS. Frontal (horizontal) section of an embryo having 

 six pairs of mesodermal somites. The section is through the notochord and just below 

 the blastopore. At the posterior end of the section may be seen a region where the 

 notochord, endoderm and mesoderm merge indistinguishably. A, archenteron near 

 the blastopore; EC, ectoderm; EN, endoderm; MES i-6, mesodermal somites; NC, 

 notochord. (After Cerfontaine.) 



from the archenteric wall which, at the place where the pouch had formed, 

 closes so that nothing is left to mark the spot. 



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 

 hatching 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 

 intact during this process of segmentation of the lateral folds, is the 

 material of the future notochord. (Fig. 42, .VC) 



