462 



EMBRYOLOGY OF THE CHICK 



Mestnchymal 

 ceil 



Fibrillae in 



fcTop/asptic 



matrix 



B 



C* i 



I 



ettof 

 wjtM 

 Elasti 





cells from both mesoderm and entoderm, and even of ectoderm, for, 

 scattered cells later join from the ectoderm of the head region. 



The somites begin forming in the region of the more anterior end 

 of the primitive streak, the first one to develop remaining the more an- 

 terior. The first four pair of somites take part in the development of 

 the hinder part of the head region of the embryo. 



A further important factor to remember at this point is that seg- 

 mentation is fundamental, 

 and that consequently any 

 structures in the body which 

 show segmentation, only fol- 

 low out some plan of the 

 original segmentation. This 

 is of value in tracing the 

 growth of various body- 

 parts, such as muscles, for 

 instance, in that the nerve 

 supply, which we shall 

 shortly see is also of seg- 

 mental origin, definitely tells 

 us where a muscle springs 

 from, because nerves always 

 follow muscles, and not vice 

 versa. 



The somatopleure, 

 splanchnopleure, and coelom 

 become separated into em- 

 bryonic and extra-embryonic 

 regions later, although at 

 this early stage of which we 

 are writing they form con- 

 tinuous structures which ex- 

 tend laterally out from the 

 germ wall, and anteriorly 

 into the head region. 

 The following structures are developed from the embryonic portion: 



body-wall, 



gut-wall, 



vascular organs, 



pericardial cavity, 



pleural cavity, 



peritoneal cavity. 



Cartilage cell 



Eciodt 

 Mts 



Fig. 269. 



Figures showing the differentiation of the supporting 

 tissues (after Mall). A, white fibers forming in the 

 dermis of a 5 cm. pig embryo ; B, elastic fibers forming 

 in the syncytium of the umbilical cord from a 7 cm. 

 embryo ; C, developing cartilage from the occipital bone 

 of a 20 mm. pig embryo. Mesenchyma from the head of 

 a thirty-six hour chick embryo. 



