620 



THE INVERTEBRATA 



The eggs are laid in the sea and develop rapidly, passing through 

 typical blastula and gastrula stages, after which the coelom is deve- 

 loped as a pair of anterolateral pouches of the archenteron (Fig. 430 A). 

 After gastrulation two cells become very prominent. These are the 

 mother cells of the generative organs. The primary coelomic cavity is 



mes. 



Fig, 429. Transverse section through middle of trunk of Sagitta bipunctata. 

 After Burfield. al.c. alimentary canal (intestine) ; gl.c. gland cells (the cells 

 which are not stippled are absorptive cells) ; lat.fn. lateral fin ; mes. mesentery ; 

 od. oviduct; ov. ovary (covered by endothelium); sp.d. sperm pouch. 



std. 

 hd-coe. 



-\-<il.c. 



Fig. 430. Larvae of Sagitta showing formation of coelomic pouches from the 

 archenteron. After Burfield. In A the pouches still open into the archen- 

 teron. In B the pouches forming the head coelom have completely separated 

 oflF from the archenteron and the archenteric folds have grown back so as 

 partly to separate off the second pair of pouches, al.c. alimentary canal ; bis. 

 blastopore; ect. ectoderm; end. endoderm; g.c. genital cells; hd.coe. head 

 coelom ; M. mouth ; std. stomodaeum. 



divided up first of all by the separation of the head cavity (Fig. 430 B) 

 and at a later stage by a second septum between trunk and tail, which 

 divides the genital cells, which now number four, into an anterior 

 pair, the mother cells of the ovaries, and a posterior pair, those of the 

 testes. 



