126 



EMBRYOLOGY. 



HOFFMANN, and others. At the front end of the primitive groove a 

 narrow canal has arisen, which leads obliquely from the neural tube 

 under the entoderm, and unites the two in the same manner in which 

 the blastopore does in Amphioxus and the Amphibia. A diagram- 

 matic longitudinal section through the hind end of a Chick (fig. 88) 

 shows us this important union (n.e), which exactly corresponds to the 



condition of' 

 an A rnphi- 

 bian embryo 

 presented in 

 fig. 80. 



Such a 

 neurenter i c 

 canal has 

 been ob- 

 served still 

 more dis- 

 tinctly in 

 Selachians 

 and Reptiles 

 and at even 

 e a r 1 i e r 

 stages, 

 whereas in 

 Teleosts it 

 special subsidiary 



Fig. 88. Diagrammatic longitudinal section through the posterior end of 

 an embryo Chick at the time of the formation of the allantois, after 

 BALFOUR. 



The section shows that the neural tube (Sp.c) is continuous at its posterior 

 end with the post-anal intestine (p.a.g) by means of the neurenteric 

 canal (n.e). The latter traverses the remnant of the primitive streak 

 ( pr), which is folded over on to the ventral side, ep. Outer germ-layer ; 

 ch, chorda ; liy, entoderm ; al, allautois ; me, middle germ-layer ; an, 

 the place where the anus will arise ; am, amnion ; so, somatopleure ; 

 sp, splanchnopleure. 



does not come to development on account of 

 conditions.* 



The investigation of the embryonic fundaments of a Mammal fur- 

 nishes us with views quite similar to those respecting the Chick. When 



* la Selachians the blastopore is very early enclosed within the medul- 

 lary folds, and then assumes the condition of a long-persisting canal-like 

 passage to the intestinal cavity through the floor of the medullary groove, 

 and later through that of the neural canal. 



In the case of Eeptiles, the primitive streak is very short and triangular, 

 and in many species soon discloses, before other organs have been differentiated, 

 an opening at its anterior end which leads to the cavity under the germ-disc, 

 which is filled with yolk. Subsequently the opening is converted into a canal, 

 the wall of which is composed of cylindrical cells, and is in continuity above 

 with the outer germ-layer, and below with the inner germ-layer. Then the 

 medullary folds, which are being formed in front of the orifice, grow around 

 it ; the orifice now becomes a genuine neurenteric canal, which in many cases 

 appears to become obliterated even before the closure of the medullary tube, 

 but in other cases persists for a long time. 



