OOGENESIS 123 



is essentially the same as that described for the spermatozoon, 

 thj difference beinjj that, whereas the two successive divisions 

 of the spermatocyte result in the formation of four spermatids, 

 each of which becomes a spermatozoon, the two successive 

 divisions of the oocyte result in the formation of four cells, 

 three of which are minute polar bodies and are to be regarded 

 as abortive ova, while the fourth is of large size, is filled with 

 deutoplasm, and is the definitive ovum, ready for fertilisation. 

 The nuclear changes, however, are identical in character. 

 The first division of the oocyte is the meiotic or heterotype 

 division, resulting in the distribution of one of the components 

 of each of the twelve bivalent chromosomes to each of the 

 cells formed by the division. There are therefore twelve 

 univalent chromosomes in the first polar body and twelve in the 

 oocyte. The second division of the oocyte is homceotypic, 

 and each of the twelve chromosomes is divided longitudinally, 

 one product of division passing into the second polar body, 

 the other remaining in the ovum. The first polar body does 

 not always divide again ; if it does the division is homceotypic, 

 and resembles the second division of a spermatocyte. In all 

 four cells then, the ovum and the three polar bodies, there are 

 twelve univalent chromosomes, just as there are in the four 

 spermatids. But the polar bodies are not destined for further 

 development : they may persist for some time under the 

 vitelline membrane but eventually disintegrate and disappear. 

 The ovum is likewise incapable of further division and de- 

 velopment unless it is stimulated thereto by fertilisation. 



Two distinct processes are involved in the act of fertilisation. 

 Firstly, the union of the sperm-nucleus with the egg-nucleus, 

 whereby the full number of chromosomes characteristic of the 

 species is restored. This process is believed to be intimately 

 connected with the transmission of the heritable qualities of 

 both parents to the offspring. Secondly a stimulus is con- 

 veyed to the ovum by the spermatozoon, under the influence 

 of which the latter divides and gives rise to a new multicellular 

 organism. 



Briefly told the phenomena of the fertilisation of the 

 amphibian ovum are as follows : The spermatozoon, including 

 head, middle piece and tail, enters the ovum at any point 

 of the pigmented hemisphere and at the point of entry a deep 

 funnel-shaped pit is formed, at the bottom of which lies the 



