ESTABLISHMEXT OF THE EXTERNAL FORM OF THE BODY. 



201 



groove is deepened into a pit, the more its ridge is turned back- 

 wards. 



Two diagrammatic longitudinal sections, one of which is shown in 

 fig. 122, the other on Plate I., fig. 11, may serve to illustrate this 

 process. 



In fig. 122 there is shown, projecting above the otherwise smooth 

 flat surface of the germ-layers, a small protuberance, which encloses 

 the anterior end of the neural tube (^V.6') and the simultaneously 

 forming intestinal tube (Z)), and which has arisen by the formation 

 of the fold F.So. The upper sheet of the fold, by directing itself 



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



BALFOUR. 

 The section represents the condition when the head-fold has begun, but the tail-fold is still 



wanting. 

 F.So, Head-fold of the soruatopleure ; F.Sp, head- fold of the splauchnopleure, forming at Sp the 



lower wall of the front end of the mesenteron ; D, cavity of the fore gut ; pp, pleuroperitoneal 



cavity ; Am, fundament of the anterior fold of the amnion ; N.C, neural tube ; Ch, chorda ; 



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



HI, heart. 



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

 lower sheet forms the floor of the marginal groove. 



In the second figure, in which there is represented a diagrammatic 

 longitudinal section through an older embryo, the head-fold (kf 1 ) has 

 extended still farther backward. The head has thereby become 

 longer, since its under surface has increased in consequence of the 

 advance in the process of folding. 



Whoever desires to make this process, 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 easily constructed 

 model. Let him stretch out his left hand on a table, and spread flat 

 over the back of it a cloth, which is to represent the blastoderm ; 

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

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

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



