Chap, VI.] GENERAL EMBRYOLOGY. 105 



and at either end, and at the sides, convert it into a tube 

 (Fig. 38, iv.). So that we have a small tubular embryo con- 

 nected by a hollow stalk with the large yolk-sac or the umbilical 

 vesicle. 



In the meanwhile the cleavage of the mesoblast is in progress. 

 The ventral and lateral plates split into two layers, of which 

 one closely adheres to the hypoblast of the digestive tube. This 

 is the splanchnic layer (sj). l>\ which, with the adjoining 

 hypoblast, forms the splanchnopleure. The other forms the inner 

 lining of the body walls. This is the somatic layer (so. L), 

 which, with the adjoining epiblast, forms the somatopleure. The 

 space between them is that of the primitive body-cavity (b. c.). 

 These are seen in Fig. 38, iii., which is a quite diagrammatic 

 transverse section of the embryo. 



In the frog there is no folding off of the embryo, and no 

 yolk-sac or umbilical vesicle, though the yolk-cells beneath the 

 mesenteron are homologous with these structures. The cleavage 

 of the mesoblast takes place as in the other types. 



The Heart, Cerebral Vesicles, and Mesoblastic Somites. It 



will be well to describe here the appearance of embryo chicks of 

 about 24, 36, and 48 hours incubation. They may be obtained 

 either from the hen or by means of an incubator kept at a tempera- 

 ture of from 38-40 C. To prepare the embryo for observation the 

 student should place the egg in a basin of normal saline warmed 

 to 38 C., and carefully break away the shell. On exposing the 

 yolk the embryo will be clearly visible on its upper surface. It 

 lies in the midst of the area pellucida, which is surrounded 

 by a mottled vascular area containing embryonic blood-vessels. 

 With sharp scissors an incision should be rapidly carried round 

 the vascular area. The blastoderm may then be floated off from 

 the yolk, and the vitelline membrane which covers it may be 

 removed. It should then be floated on to a glass slide and 

 covered with a cover-glass, taking care to have plenty of normal 

 saline. 



In Fig. 37, i., the embryo (of about 24 hours) is seen, from 

 the ventral aspect, in the midst of the area pellucida. Anteriorly, 



