DEVELOPMENT. 



267 



Arthropoda. The spherical ovum is surrounded by a cuticular vitelline 

 membrane, and contains a considerable quantity of yolk. After ferti- 

 lisation the segmentation nucleus divides in the usual way into 2, 4, 

 8, and so on, but this nuclear division is not followed by division of the 

 plasma. Eventually the nuclei, each surrounded by a small amount of 

 protoplasm, approach the surface of the egg and arrange themselves 

 regularly round it. The peripheral protoplasm then segments round 

 these nuclei, and thus we have a central core of unsegmented yolk 

 enveloped by a peripheral ring of rapidly dividing cells. In the central 

 yolk free nuclei may be frequently found, these are the so-called yolk 



Fig. 87. — Longitudinal section of later embryo of 

 Astacus. (After Reichenbach.) 



ec, Ectoderm ; in, mesodernt cells ; eg, cerebral ganglia ; st, stoma- 

 todaeum ; A , anus : T, Lelson ; ff, ventral ganglia ; ss, sternal sinus ; 

 id, proctodseum ; h, heart ; 7ug, mid gut ; yolk pyramids (dark). 



nuclei. Such a type of segmentation is called peripheral or centro- 

 lecithal, and is very characteristic of Arthropod eggs. 



Over a particular region of the segmented egg, known as the " ventral 

 plate," the cells begin to thicken ; at this region an invagination occurs, 

 which represents the gastrula. At the anterior lip of the blastopore the 

 mesoderm appears, being many celled from the first. Soon the blasto- 

 pore closes ; the cavity of the gastrula thus becomes a closed sac — the 

 future mid gut. The cells of this archenteron take up the core of yolk 



