GENERA TI VE ORGA NS. 



G13 



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 them- 

 selves 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 di- 

 viding the tail from the 

 body, the ovarian cells lie 

 immediately in front of 

 this septum, and the testi- 

 cular cells in the region 

 behind it. 



Polyzoa. In Pedicel- 

 lina amongst the entc- 

 proctous Polyzoa, Hatschek 



finds that the generative 



onrans originate 



from 



a 



FIG. 409. Two VIEWS OF A LATE EJIBKYO OF 

 SAGITTA. A, from the dorsal surface. B, from the 

 side. (After Btitschli.) 



m. mouth; al. alimentary canal ; i'.g. ventral 

 ganglion (thickening of epiblast) ; ep. epihlast ; 



- c.pv. cephalic section of body cavity; so. somato- 



pair of specially large me- pleure; sp. splanchnopleure ; //('.generative organs, 

 soblast 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 Cha^tognatha. They become surrounded 

 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 meso- 

 blastic 1 . 



Insecta. The generative cells have been observed at a very 

 early embryonic stage in several insect forms (Vol. I. p. 344), 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 2 has shewn 

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



1 Vide Vol. i. p. 309; also Gotte, Zool. Anzeiger, No. So, p. 189. 



2 C. Grobben. "Die Eutwick. d. Moina rectirostris." Arbeit, a. d. zool. Instit* 

 Wien. Vol. n. 1879. 



