266 COMPARATIVE ANATOMY 



posterior third of the body. The testes, generally two or three 

 and frequently more in number, are borne on the upper third 

 of the body wall, rarely extending down to the middle and 

 posterior thirds. 



An ovary originates as an ectodermic swelling caused by 

 the rapid multiplication of indifferent interstitial cells. In this 

 manner a mass of primitive ova or oogonia is formed, but of 

 these only one undergoes further development and becomes an 

 oocyte. (See p. 117.) The remainder are arrested in growth 

 and go to form food material for the oocyte, which increases 

 greatly in size, takes up a central position amongst its fellows, 

 and begins to emit pseudopodial processes. As it grows it 

 seizes upon and ingests the arrested oogonia as an Amoeba 

 ingests its prey, storing up their digested products in the form 

 of a number of dark spherical corpuscles, which have been 

 called pseudocells, but which we had better call yolk corpuscles 

 or deutoplasts. Whilst these processes are going on they 

 last for several days the ectoderm cells in the neighbourhood 

 of the developing ovary swell up and form a covering for the 

 oocyte and the oogonia or reserve cells on which it feeds. By 

 the time that all the reserve cells are eaten up, the ovum has 

 increased enormously in size and the ectoderm cells covering 

 it are stretched till they form only a thin envelope around it. 

 The ovum withdraws its pseudopodia, assumes a hemispherical 

 shape, and by two successive unequal divisions gives rise to 

 two polar bodies. Whilst these are being formed a structure- 

 less gelatinoid substance is secreted between the ovum and its 

 covering cells. After the formation of the polar bodies the 

 ovum becomes quite spherical except for a flattened area where 

 it rests upon the mesoglcea. The covering cells become very 

 thin, break at the distal pole of the ovum, and shrink back- 

 wards towards its base, leaving its protoplasm naked except 

 for an envelope of the gelatinoid material secreted at the 

 previous stage. (Fig. 56, A.} The ovum is now ready for 

 fertilisation, which is effected in the normal manner, by the 

 entrance of a single spermatozoon. 



A testis originates in the same way as an ovary viz. as a 

 proliferation of indifferent interstitial cells of the ectoderm 

 giving rise to a little mass of germ cells spermatogonia. The 

 mass is enclosed by several epithelio-muscular covering cells, 

 whose expanded outer ends form the wall of the testis, their 



