REPRODUCTION 



87 



relieve the perplexity occasioned by the variability in the development 



of the organ in vertebrates. In Amphioxus the anterior part of the 



notochord is folded up from the endo- 



derm but a considerable posterior extent 



of it is built on by proliferation at 



the blastoporal region. The primitive 



streak of the amniote embryo is open to 



interpretation as being a modified 



blastoporal region. The streak is then 



the equivalent of the blastoporal rim, 



both actively growing regions. If all 



this be granted, then the lower verte- 



(From Kingsley, Comparative Anat- 

 omy of Vertebrates.) 



Fig. 59. — Stereogram of embryonic 

 neural tube showing the segmenting 

 neural crest, e, superficial ectoderm; 

 brate (amphibian) retains more of the nc, neural crest; s, central canal. 



Amphioxus method of forming noto- 

 chord while in higher vertebrates there 

 is less of direct and obvious origin from endoderm, and proliferation 

 from the blastoporal streak plays the main role — all of which seems 



quite reasonable. 



The Enteron. Gastrulation produces 

 a two-layered embryo whose endoderm 

 surrounds a cavity opening to the exterior 

 by the blastopore. This archenteric 

 cavity is the prospective digestive cavity. 

 As the embryo elongates, the cavity is 

 correspondingly elongated and in later 

 development the enteric tube increases in 

 length faster than the embryo with result' 

 that the tube becomes bent or even coiled 

 to adapt itself to the coelomic space. 



In the early embryo the ectoderm at a 

 median antero-ventral position gives rise 

 to a shallow depression or pit, the 

 stomodeum, whose deeper wall meets the 

 forward-growing endoderm to form tem- 

 porarily a two-layered oral membrane 

 separating the external stomodeal cavity 

 from the enteric cavity. Soon a perfora- 

 tion appears at the center of the mem- 

 brane (Fig. 61, 0) and its peripheral 

 remnant is rapidly obliterated. The 

 perforation and obliteration of the membrane apparently result from pro- 

 gressive centrifugal flow or movement of its cellular substance. Thus 

 is formed the mouth. The posterior enteric aperture or embryonic 



Fig. 60.- — Stereogram of the 

 developing eye. The head of 

 the embryo is cut transversely 

 in the region of the fore-brain. 

 cf, choroid fissure; fb, wall of 

 fore-brain; /, ectodermal thicken- 

 ing which invaginates to form 

 lens; oc, optic cup; os, optic stalk; 

 p, outer thin wall of optic cup, 

 becoraing the pigmented epithe- 

 lium which lies behind the defini- 

 tive retina; r, inner thick wall of 

 optic cup, becoming the sensory 

 retina of the eye. (From Kings- 

 ley, Comparative Anatomy of 

 Vertebrates.) 



