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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



to the surface of the ovum, and part of it, together with a minute quantity 

 of protoplasm, is pinched off as the first polar cell (F, L, G). A second 

 division (H, I, K) leads to the production of a second polar cell, and the 

 nucleus, now reduced by three-fourths, travels back to the centre of the 

 ovum. It is now termed the female pronudeus, and does not contain 

 enough chromatin to enable the ovum to develop further. In certain ova 

 which can develop without fertilization (i.e., are parthenogenetic) only one 

 polar cell is formed. Weismann supposes that the germinal vesicle contains 

 two kinds of protoplasm, (1) ovogenetic substance, which presides over the 

 growth of the ovum, and (2) germ-plasma, which enables it to develop into 

 an embryo. He believes the first polar cell (or only one in cases of par- 

 thenogenesis) to consist of (1), the second of half (2). A certain amount of 

 germ-plasma is imagined to be requisite for development, and when two 

 polar cells are formed too little is left for the purpose. Fertilization 

 according to this would seem to mean the importation of germ-plasma in 

 sufficient quantity to make up the deficit. In Ascaris the polar cells are 

 not formed till after the entry of the sperm. 



In spermatogenesis something akin to the formation of polar cells is to 

 be observed. The whole of a mother-sperm-cell does not become converted 

 into sperms; there is always a residue forming a sperm-blastophor or an 

 equivalent to it. A typical sperm is somewhat tadpole-shaped. The head 

 contains a nucleus rich in chromatin, and covered by a thin layer of pro- 

 toplasm drawn out into the vibratile tail, by means of which the sperm is 

 propelled head first. 



Fig. 22. FERTILIZATION OF OVUM OF A STARFISH (Asterias glacialis) 

 (after Fol). In A-D the sperms are represented as imbedded in the 

 mucilaginous coat of the ovum. In A a small prominence is rising 

 from the surface of the ovum towards the nearest sperm. In B they 

 have nearly met, and in C they have met. D, The sperm has pene- 

 trated the ovum, and a vitelline membrane which prevents the entry of 

 other sperms has been formed. H, Ovum showing polar cells and 

 approach of the $ and $ pronuclei ; the protoplasm is radially 

 striated round the former. E, F, G, Later stages in the coalescence of 

 the two pronuclei. 



Fig. 22 illustrates fertilization (impregnation) as it occurs in a starfish 

 (Asterias glacialis). A single sperm penetrates the ovum, with the proto- 

 plasm of which its protoplasm fuses, while its nucleus, now known as the 

 male pronudeus, unites with the female pronucleus to form the segmentation- 

 nucleus. The oosperm constituted by the union of the male and female 

 cells can now develop into an embryo. The phenomena of fertilization 

 have been studied with great care in Ascaris, and it has been shown that 



