REPRODUCTIVE ORGANS. 65 



function, two kinds of accessory organs, solid particles sus- 

 pended in a fluid and fine hair-like filaments, are developed 

 in close relation with the nerve-endings. In the Crustacea 

 both are combined, and an involution of the sensory region 

 takes place, which usually remains open throughout life, and 

 represents the most rudimentary form of auditory labyrinth. 

 The Crustacean ear is the parallel of the Nautilus eye. In 

 the Vertebrata the membranous labyrinth is similarly an in- 

 volution of the integument, which remains open throughout 

 life in many fishes, but becomes shut off and surrounded by 

 thick mesoblastic structures in all the higher Vertebrata. 

 The tympanum and the ossicula auditus are additional 

 accessory structures, formed at the expense of the hyoman- 

 dibular cleft and its boundary-walls. 



The Reproductive System. — The relation of the reproduc- 

 tive elements to the primitive layers of the germ is as yet 

 uncertain. E. van Beneden has brought forward very strong 

 evidence to the effect that in Hydractinia the spermatozoa 

 are modified cells of the ectoderm, and the ova of those of the 

 endoderm ; but, whether it can be safely concluded that this 

 rule holds good for animals generally, is a question that can 

 only be settled by much and difficult investigation. The fact 

 that, in the Vertebrata, the ova and spermatozoa are products 

 of the epithelial lining of the peritoneal cavity, and therefore 

 proceed from the mesoblast, appears at first sight directly to 

 negative any such generalization. But it must be remem- 

 bered that the origin of the mesoblast itself is yet uncertain, 

 and that it is quite possible that one portion of that layer may 

 originate in the ectoderm and another in the endoderm. 



There is some reason to suspect that hermaphrodism was 

 the primitive condition of the sexual apparatus, and that uni- 

 sexuality is the result of the abortion of the organs of the other 

 sex, in males and females respectively. 



Very low down in the animal series, among the Turbella- 

 ria, the accessory organs of generation acquire a great com- 

 plexity. In the lower Turbellaria the excretory duct is a 

 mere short, wide passage. But, in the higher Turbellaria and 

 Trematoda, the female apparatus presents a germarium, in 

 which the ova are developed ; vitellarian glands, which give 

 rise to a supplemental or food yelk ; an oviduct ; a uterus and 

 vagina ; and a spermatheca, in which the semen is stored up. 

 The male apparatus presents a testis, a vas deferens, and a 

 penis. The function of the vitellarian gland may be taken on 



