iBLISHMENT OF ' III 



201 



deepen* .1 into a pit. the turn, d i 



wards. 



TWO diagrammatic longitudinal .f whi<-h i- >!:->wn ill 



\'2'2. the other on Plato 1.. tiur. 11. may this 



process. 



In li,'. 1-- there i- shmvn. projecting above tin- otherwise smooth 



tlat rarfaoe of the germ-layers, a -mall protul. ranc<-. which encloeee 

 the anterior end of the neural tube (3V) and the simul< 

 forming intestinal tube (/)), and which ha- ari-en by the formation 

 of the fold F.So. The upper sheet of the fold, l.y ! 





Fig. 122. -Diagrammatic longitudinal section through the axis of an embryo Bird 

 BALFOUR. 



The section represents the condition \vln-n tin- hi-.-ul-fuld h.i* begun, but the t.iil -il>l ia still 

 Banting. 



F.So, Head-fold of the somatopleure ; F.Sp, head folU of the splauclinopleure, forming at Sp the 

 lower wall of the front end of the oiesenteron ; D, c.tvity of the fore gut ; ;/. -.meal 



cavity ; Am, fundament of the anterior fold of the amnion ; A'.C 1 , neural tube ; Ch, ch 

 A, B, C, outer, middle, inner germ-layer, everywhere distinguished by different sin 

 lit, ! 



backwards, furnishes the ventral wall of the cephalic elevation ; the 

 1< ur sheet forms the floor of the marginal groove. 



In the second figure, in \vhich there is represented a diagrammatic 

 longitudinal section through an older embryo, the head-fold (A;/* 1 ) has 

 extended still farther backward. The head has thereby become 

 longer, simc its under surface has inert ased in consequence of the 

 advance in the process of folding. 



Whoever desires to make this proiv. which is very important for 

 the comprehension of the construction of animal forms, clearer and 

 more intelligible, may do so with the help of an ea>il\ con-tiuct. <1 

 model. Let him stretch out his left hai.l on a tahle, and spn ad tlat 

 over the back of it a cloth, which is to ivpr->-.-nt the blastoderm ; 

 then let him fold in the cloth with his riirht hand by tucking it a 

 little way under the points of his left tinkers. The artificially pro- 

 duced fold corresponds to the head-foM previously described. The 



