608 COMPARATIVE ANATOMY. 



the embryo, and is connected with the primitive cavity of the 

 pelvic portion of the enteron (urachus). This is gradually con- 

 verted into a fusiform widened organ — the urinary bladder, 

 while the continuation of the urachus into the umbilicus, and from 

 thence into the umbilical chord, is obliterated. The former portion 

 forms the ligamentum vesico-umbilicale medium. The primitive 

 (fusiform) form of the urinary bladder is retained in some Mammals 

 (Seals), while in others it is gradually modified, and with these 

 modifications there are correlated differences in the way in which 

 the ureters open. Thus in many Rodents they open high up on 

 the posterior wall of the bladder (Fig. 354, C, 11). 



The other characters of the efferent ducts are common to them 

 and the generative apparatus, with which, therefore, they will be 

 treated. 



Generative Organs. 



§ 450. 



In the Vertebrata, the reproductive organs are shared by 

 different individuals ; the separation of the sexes is the rule, 

 although there are various exceptions to it in the class Pisces. 

 In the higher divisions also there are various arrangements which 

 are indications of hermaphroditism. But it seems to me that the 

 point which is of real importance in this matter is the repro- 

 ductive material, and that the characters of the efferent ducts are 

 of no importance, for they were not primitively part of the gene- 

 rative apparatus. 



Our knowledge of the earliest development of the male generative 

 matter is not quite definite, but we know certainly that the female 

 elements are derived from the epithelial layer which invests the 

 abdominal cavity. In this there are points of likeness between 

 the Vertebrata, and the Vermes among the Invertebrata. In 

 Amphioxus follicular structures, covered by a layer of epithelium, 

 and forming diverticula of it, are developed at various points in 

 the coelom, or in the cavities connected with it; these structures 

 are the germ-glands. The ova are formed in them, between 

 indifferent and flattened cells, which form the stroma of the organ. 

 In this character Amphioxus is very different to the Craniota, where 

 the germ-glands are always developed at a sharply defined and 

 less extensive region. The epithelial investment of the abdominal 

 cavity retains its primitive character along a tract which corre- 

 sponds to the rudiment of the primitive kidney longer than it does 

 in other regions ; and this epithelial layer may be distinguished as 

 the germinal epithelium (Fig. 346, e). At the side of the 

 mesentery in this region there is an elevation of varying length, 

 which is formed by a thickening of the connective tissue — the 

 genital ridge. The epithelium dips into this, and forms the rudi- 

 ments of the ova. Of a group of cells which grows inwards, one 



