it8 COMPARATIVE ANATOMY 



the oviducts the ova obtain gelatinous coats secreted by the 

 glands with which the oviducal 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 gelatinous investment 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 cul- 

 minating in the adult frog. 



The spermatozoa of the male are formed from the follicles 

 containing primitive ova. These become converted into the 

 seminiferous tubules of the adult testis. Briefly told, the 

 history of the primitive ova in the male is this. They become 

 spermatogonia, which divide and sub-divide, till, after a time, 

 they come to rest and increase in size. They are now known 

 as spermatocytes. Each spermatocyte divides into two, and 

 each of these again divides into two, making four spermatids 

 derived from each spermatocyte. Each spermatid is directly 

 changed into a spermatozoon. 



The changes accompanying the maturation of the ovum 

 are rather complicated in the frog, and the formation of the 

 spermatozoa is not very well understood, so it will be better 

 to describe the simplest possible case of spermatogenesis and 

 maturation of the ovum as it occurs in the round worm of the 

 horse, Ascaris megalocephala. It has already been stated 

 that this animal, or at least that variety of it known as 

 univalens, has only two chromosomata. Another variety 

 known as bivalens has four chromosomata in its nuclei, and 

 this will be chosen for the description of the processes of 

 maturation of the germ-cells. Beginning with the sper- 

 matozoon it must be explained that the spermatozoa of the 

 thread-worms (Nematoidea), of which Ascaris is an example, 

 are not filamentous, like those of most other animals, but are 

 simply cellular, and therefore are very suitable objects for study. 

 The primitive germ-cells have, like the rest of the tissue-cells of 

 the body, four chromosomata each. They multiply by division 

 again and again, the nucleus each time undergoing normal 



