REPRODUCTION 



59 



plate into a tube, the blastopore is left in communication with the lumen 

 of the tube. At its anterior end the closure of the neural tube is delayed 

 so that for a time its lumen is open to the exterior by a small aperture, 

 the neuropore. The extraordinary result of these changes is an embryo 

 whose prospective digestive cavity, still devoid of definitive mouth and 

 anus, communicates via the neurenteric canal (the former blastopore) 

 with the hind end of the cavity of the prospective spinal cord and thence 

 to the outside by the anterior neuropore (Fig. 5i,P). 



These relations, however, are merely temporary. Eventually neuro- 

 pore and neurenteric canal close. The definitive enteric apertures, 

 mouth, gill clefts and anus, arise by very similar processes. At the 



UP ,NC 



Fig. 51. — AMPHIOXUS. Median longitudinal section of an embryo having 

 two mesodermal pouches, a stage approximately like that of the transverse section in 

 Fig. 42E. The blastopore, roofed over by ectoderm, has become the neurenteric canal. 

 .4, archenteron; EC, ectoderm; EN, endoderm; NC, endoderm destined to become 

 notochord; NE, neurenteric canal; NP, neural plate; P, neuropore. X350. (Based 

 on a figure by Hatschek.) 



appropriate locality enteric endoderm and superficial ectoderm approach 

 one another and coalesce. The resulting double layer then thins out 

 until perforation occurs. 



The notochord, whose development is initiated by an upward folding 

 of mid-dorsal endoderm (Fig. 42D-F), early becomes detached from 

 the enteric endoderm and acquires its characteristic cylindrical form. 

 The enteric endoderm meanwhile closes in beneath the notochord and 

 restores the integrity of the dorsal wall of the enteron (Fig. 42G). As 

 the embryo increases in length the notochord grows within itself and 

 receives accessions from the active blastoporal region with which its 

 posterior end remains for some time connected (Fig. 43). 



The more anterior mesodermal pouches (or somites), soon after their 

 formation and long before the more posterior somites have been developed, 

 begin to acquire their characteristic differentiation. The pouch expands, 

 especially ventralwards, and its cavity is correspondingly enlarged. That 

 part of its wall lying against the notochord becomes much thickened 



