266 



ANIMAL BIOLOGY 



Blastocoel 



B 



Mesoblast 

 cell 



Ectoderm 



Mesoblast 

 cell 



Mesoblast 

 cell 



Mesoblast 

 cell 



Enteron 



Mesoblast 

 cell 



Ectoderm 



Anus 



Endoderm 



Mesoderm 



Ectoderm 



Mesoderm 



Ectoderm 



Endoderm 



Nerve 

 cord 



Somatic 

 mesoderm 



Coelom 



Splanchnic 

 mesoderm 



Endoderm 



Nerve 

 cord 



Ectoderm 



Mesoderm 



Coelom 



Endoderm 



Mouth 



Fig. 173. — Diagrams of stages in the development of the Earthworm. 

 A, blastula (surrounded by a membrane); B, section of a blastula showing 

 blastocoel and one of the primary cells (mesoblast cells) of the mesoderm; 

 C, later blastula with developing mesoderm bands; D, start of gastrulation; 

 E, lateral view of gastrula showing invagination, which as it proceeds leaves 

 the mesoderm bands on either side of the body as indicated by the cells repre- 

 sented with dotted outline; F, section of E, to show mesoblast cells, meso- 

 derm bands, and enteric cavity. G, later stage showing cavities in the meso- 

 derm bands; H, the same (G) in transverse section; I, diagram of a longitudinal 

 section of a young worm after formation of mouth and anus; J, the same in 

 cross section; K, later stage in transverse section. (After Sedgwick and W ilson.) 



