GENERATIVE ORGANS. 745 



When the archenteric cavity is divided into a median 

 alimentary tract, and two lateral sections forming the body 

 cavity, the generative organs are placed in the common vestibule 

 into which both the body cavity and alimentary cavity at first 

 open (fig. 408). 



The generative organs long retain their character as simple 

 cells. Eventually (fig. 409) the two ovaries travel forwards, and 

 apply themselves to the body walls, while the two testes also 

 become separated by a backward prolongation of the median 

 alimentary tract. 



On the formation of the transverse septum dividing the tail 

 from the body, the ovarian cells lie immediately in front of this 

 septum, and the testicular cells in the region behind it. 



Polyzoa. In Pedicellina amongst the entoproctous Polyzoa 

 Hatschek finds that the generative organs originate from a pair 

 of specially large mesoblast cells, situated in the space between 

 the stomach and the floor of the vestibule. The two cells 

 undergo changes, which have an obvious resemblance to those of 

 the generative cells of the Chaetognatha. They become sur- 

 rounded by an investment of mesoblast cells, and divide so as to 

 form two masses. Each of these masses at a later period 

 separates into an anterior and a posterior part. The former 

 becomes the ovary, the latter the testis. 



Nematoda. In the Nematoda the generative organs are 

 derived from the division of a single cell which would appear to 

 be mesoblastic 1 . 



Insecta. The generative cells have been observed at a very 

 early embryonic stage in several insect forms (Vol. II. p. 404), but 

 the observations so far recorded with reference to them do not 

 enable us to determine with certainty from which of the germinal 

 layers they are derived. 



Crustacea. In Moina, one of the Cladocera, Grobben" has 

 shewn that the generative organs are derived from a single cell, 

 which becomes differentiated during the segmentation. This 

 cell, which is in close contiguity with the cells from which both 

 the mesoblast and hypoblast originate, subsequently divides; 



1 Vide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189. 



2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zooL Instil. 

 Wien. Vol. II. 1879. 



