336 GENERAL EMBRYOLOGY 



teron, is known as imagination, and in some of the lower forms 

 the endoderm is wholly formed by this process. The process 

 of inturning, such as results in the lining of the dorsal region 

 of the archenteron by the cells derived from the margin of the 

 blastopore, is known as involution. In some forms the endo- 

 derm is largely formed by this process. Usually this is accom- 

 panied by the growth of the margin of the blastopore, or germ 

 ring, on over and past the involuted region, so that a layer of 

 cells is continually being overgrown, leading to more extended 

 gastrulation; this process of overgrowth may be termed epiboly. 

 The blastopore of the Amphioxus gastrula, at first widely 

 open, soon closes rapidly on account of the growth and epiboly 

 of its lip. This process leads also to the rapid elongation of the 

 gastrula (Fig. 151, F-H). 



Although we have included involution and epiboly as gas- 

 trulation processes they are here more properly to be regarded 

 as processes leading to the formation of the notochord and the 

 intermediate layer. For as the gastrula continues to elongate 

 these structures begin to differentiate in the anterior part of the 

 roof of the archenteron, from that part of the inner layer formed 

 by involution and epiboly. Along the dorso-lateral regions of 

 the archenteron appear a pair of folds out of the archenteron, 

 each containing a narrow groove (Fig. 152, B, C). These folds 

 are the rudiments of the intermediate layer, or mesoderm, or meso- 

 blast; and the grooves, known as the enteroccelic grooves, are the 

 rudiments of the ccelom, the cavity of the mesoderm. That 

 portion of the archenteric roof between the mesoderm folds, 

 later becomes folded outward and forms the rudiment of the 

 notochord. The remainder of the archenteric wall, namely the 

 ventral and ventro-lateral regions formed by the invaginated 

 portion of the inner layer, forms the primitive gut or enteron. 

 Whether we choose to call the involuted region really endoderm 

 or not, is immaterial; if we do, then we must say that the chorda 



established by overgrowth of neural folds. Continued mitosis in germ ring, 

 a, animal pole; ar, archenteron; b, blastoporal opening; ch, rudiment of noto- 

 chord; dl, dorsal lip of blastopore; ec, ectoderm; en, endoderm; gr, germ ring; 

 nc, neuenteric canal; nf, neural fold; np, neural plate; s, blastocoel or segmenta- 

 tion cavity; v, vegetal pole; vl, ventral lip of blastopore; //, second polar body. 



