n8 COMPARATIVE ANATOMY 



the first post-meiotic division. The two small cells formed 

 by the successive divisions of the oocyte are known as polar 

 bodies. While the maturation is in process the ova are dis- 

 charged into the body-cavity by the bursting of their follicles ; 

 they are passed along partly by muscular contractions of the 

 abdominal walls, partly by ciliary action, to the mouths of the 

 oviducts, and travel down the oviducts into the uteri. During 

 its passage down the oviducts each ovum is invested by a coat 

 of slime or mucin secreted by the glands with which the ovi- 

 ducal walls are abundantly supplied. The ova accumulate in 

 great numbers in the highly distensible uteri, and eventually are 

 expelled through the cloaca into the water, in which the mucin in- 

 vestment swells up to form the well-known jelly of frog's spawn. 



At the time of their passage to the exterior the ova are 

 fertilised by the simultaneous discharge of the semen of the 

 male frog. Without fertilisation they are incapable of further 

 development ; but if fertilisation takes place each ovum goes 

 through a series of divisions which result in the formation 

 of a multi-cellular aggregate, leading up to the tadpole and 

 culminating in the adult frog. 



The spermatozoa of the male are derived from the cell- 

 clusters described above. At first the cells of each cluster 

 are all alike, but as development proceeds those more centrally 

 situated become spermatogonia and those more peripherally 

 situated form a sac or follicle investing them. The spermato- 

 gonia divide and subdivide, with normal pre-meiotic nuclear 

 mitoses, till, after a time, they come to rest and increase in 

 size. They are now known as spermatocytes. Each spermato- 

 cyte divides equally into two, and each of these again divides 

 into two, making four spermatids derived from each spermato- 

 cyte. In the first division the nuclear mitosis is of the hetero- 

 type or meiotic kind : in the second division it is of the 

 homoeotype or first post-meiotic kind. Each spermatid is 

 directly changed into a spermatozoon. The spermatozoon of 

 the frog consists of ( r) a head, which differs somewhat in shape 

 in different species of frogs ; (2) a minute segment immediately 

 behind the head, which appears to consist of cytoplasm with 

 a contained centrosome ; (3) a whiplash-like tail or flagellum 

 by whose movements the spermatozoon is propelled through 

 liquid. The head consists entirely of nuclear matter, except for 

 a very thin layer of cytoplasm which forms an investment to it. 



