THE EMBRYO ON THE SECOND DAY. 



475 



cut off by longitudinal and transverse fissures. They develop 

 in order from before backwards, beginning at the level of 

 the ear. They give rise to all voluntary muscles, except those 

 of the head, the axial skeleton of the body, and the dermis. 



Fig. 85. A diagrammatic figure of the blastoderm of a Hen's Egg about 

 the twenty -fourth hour of incubation. (In part after Duval.) x 8. 



AD, area pellucida : the part left white is the proamnion, and consists of 

 epiblast and hypoblast alone : in the hinder part of the pyriform area pellucida, 

 covered by the light shading, mesoblast is present as well. AK, area opaca. 

 BF, commencing fore-brain. M, dotted line indicating the limit to which the 

 mesoblast has spread. MS, mesoblastic somite or protovertebra. NG, neural 

 groove. PS, primitive streak. 



Transverse sections of a two-day chick show (fig. 90, MP) 

 that these somites are at first hollow structures. Very 

 shortly afterwards, by a horizontal slit in the lateral wings 

 of the primitive streak, the mesoblast from which they are 

 cut off becomes hollow too. This latter cavity forms the 

 * ccelom ' or body-cavity of the future chick. The outer wall 

 of the cavity forms the body wall of the chick, and is there- 

 fore called the ' somatopleure,' whilst the inner wall, at pre- 

 sent stretched over the yolk, will form the wall of the gut or 

 1 splanchnopleure.' The hypoblast, meanwhile, has become a 



