214 



parallel genital ridges of the dorsal lining of the splanchno- 

 coele, near where the dorsal mesentery is attached. The 

 testis consists of numerous seminiferous tubules amidst a 

 stroma of vascular connective tissue (derived from mesen- 

 chyme). The tubules are continuous with tubular out- 

 growths (vasa efferentia) of the nephrotomes of the meso- 

 nephros ; and thus the spermatozoa pass into the meso- 

 nephric or Wolffian duct, which acts as a male duct or 

 vas deferens. The ovary has a similar nutritive and sup- 

 porting stroma, amidst which there are numerous follicles. 

 One of the cells in each follicle becomes an ovum, the 

 others form an envelope around it. When this Graafian 

 follicle bursts, the ovum is discharged, generally into the 

 body cavity ; thence it enters the coelomic funnel of an 

 oviduct (Mullerian duct), and is either passed out, or under- 

 goes development within the uterine region of the oviduct. 

 The ova may be fertilised outside the body (e.g., Teleostean 

 fishes, tailless Amphibians), or internally (eg., El as mo branch 

 fishes, Reptiles, Birds, Mammals). Hermaphroditism occurs 

 normally in a few Craniates. 



The amount of yolk in the ova or eggs of Craniates is 

 variable, and greatly influences development. The ova 

 of Elasmobranch and Teleostean fishes, and of Reptiles, 

 Birds and Monotreme Mammals have a great quantity of 

 yolk ; consequently segmentation is partial (meroblastic). 

 In these groups the young, when hatched out, resemble 

 the parents, except Teleosteans, which are hatched in an 

 immature state (larva). The ova of certain Fishes (e.g., 

 Ganoids and Dipnoi) and of Amphibians have a consider- 

 able quantity of yolk, and segmentation is complete (holo- 

 blastic) but unequal. In these groups the young are hatched 

 as larvae. The ova of most Mammals have little yolk, 

 and segmentation is complete (holoblastic) and equal. 



Around the embryos of Reptiles, Birds, and Mammals 

 there are protective and respiratory and nutritive foetal 

 membranes (amnion and allantois)/ During gestation, in 

 Mammals, there is a close (placental) connection between 

 the foetus and the mother ; and the young*, when born, 

 plosely resemble the adult t 



