COMPARATIVE ANIMAL MORPHOLOGY AND PHYSIOLOGY. 317 



a rudimentary allantois, and in the other is the proximal end 

 of that structure, and receives the urinary ducts. 



4. Reproduction. This is effected asexually in Amoeba and 

 Vorticella by means of fission, and in Hydra by means of gem- 

 mation. 



Sexual Reproduction is first hinted at in conjugation, which 

 differs somewhat from the same phenomenon as exhibited by 

 plants, in that it is merely a temporary or permanent (as in 

 Vorticella) fusion of two similar or dissimilar individuals which 

 leads to more vigorous asexual reproduction. In Gregarina it is 

 followed by spore-formation. 



In Hydra both male and female reproductive organs, spermaries 

 and ovaries, producing male and female reproductive cells (sperms 

 and ova) respectively, are present in the same individual. Fer- 

 tilization is, probably, preceded in all cases by the formation of 

 two polar cells from the ovum, and in most forms at any rate the 

 germinal cells producing sperms are not entirely used up in the 

 process. 



The spermaries and ovaries may be termed the essential organs, 

 and advance upon the conditions seen in Hydra principally con- 

 sists in the development of various accessory parts, such as ducts, 

 special glands, and copulatory organs. Amphioxus, however^ 

 possesses no such supplementary structures. In Dogfish and 

 Frog the mesonephros performs, in the male, a double function. 

 In the Pigeon and Rabbit the mesonephros aborts, but its duct 

 remains in the male as the spermiduct. A cloaca is present in 

 the Dogfish, Frog, and Pigeon, but, in the Rabbit, the termination 

 of the alimentary canal is quite distinct from that of the urino- 

 genital organs, and opens to the exterior by an anus, while they 

 do so by an urinogenital opening. 



Considerable variations are presented in development, which 

 are largely dependent upon the amount of food-yolk in the egg. 

 In the Rabbit this is present in but very small quantity, and the 

 nutritive material and oxygen required by the embryo are supplied 

 by the mother, the embryonic and maternal blood-vessels becoming 

 closely related in the placenta. Waste is partly carried off in the 

 same manner. 



Hydra differs markedly from the higher forms, in that two 

 germinal layers only, ectoderm and endoderm, are present. This 

 animal is in fact little more than a permanent gastrula. The 



