238 COMPARATIVE ANATOMY 



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 

 elongated bodies forming columns which traverse the mass of 

 spermatogonia and imperfectly divide it up into compartments. 

 The spermatogonia develop into spermatocytes, and each 

 spermatocyte divides to form four spermatids, which develop 

 directly into spermatozoa. (See p. 118.) A spermatozoon of 

 Hydra has a conical head, consisting almost entirely of nuclear 

 matter, a small protoplasmic middle-piece, and a fairly long 

 vibratile flagellum. 



After fertilisation, the ovum divides, still remaining attached 

 by a short stalk or foot to the parent. The segmentation is 

 total and regular. Two successive meridional divisions divide 

 the ovum into blastomeres, and the next equatorial division 

 produces an eight-celled stage. At this period, a space — the 

 segmentation cavity or blastocoele — appears between the inner 

 ends of the blastomeres. The final result of segmentation is a 

 hollow sphere or blastula, having a wall composed of a single 

 layer of cells surrounding a central cavity, the blastoccEle. 

 (Fig. 50, £.) 



It is a striking fact that in two animals so far apart in the 

 animal scale as the frog and Hydra the result of segmentation 

 is the same — viz. the formation of a blastula. 



The next step is the formation of a two-layered embryo, 

 which is effected in Hydra by a process known as multipolar 

 immigration. During the growth of the blastula the planes of 

 division of the blastomeres were radial, but now several of the 

 cells undergo tangential divisions and the innermost of their 

 products pass into the blastocoele. Other cells, again, slip 

 bodily from their positions in the blastula wall and pass into 

 the blastocoele, and eventually the latter cavity is completely 

 filled up by cells which have been derived by these two 

 methods from the blastula wall. Immediately after this the 

 outermost layer of cells — i.e. those of the blastula wall — 

 divide rapidly by radial divisions and form a definite layer of 

 columnar epithelium sharply marked off from the inner mass of 

 cells. (Fig. 50, D, ec.) This outer layer may now be called the 

 ectoderm, the inner mass the endoderm. 



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