DESCRIPTION OP THE SEXUAL PRODUCTS. 27 



tail, and to know their different chemical and physical properties. The view 

 expressed by KSllikee, that ordinarily the seminal filaments were the 

 metamorphosed and elongated nuclei of the seminal cells, underwent a modifi- 

 cation. According to the researches of La Valette, only the head of the 

 seminal filament arises from the nucleus, the tail, on the contrary, from the 

 protoplasm of the spermatid. Finally Flemming brought forward convincing 

 proof that it is only the chromatin of the nucleus that is metamorphosed into 

 the head of the seminal filament. Important investigations concerning the 

 development of the seminal filaments in various animals have recently been 

 made by van Benedbn et Julin, Platnee, Hermann, and others. 



Summary. 



The most importaiit results of this chapter may be briefly sum- 

 marised as follows : — 



1. Male and female sexual products are simple cells. 



2. The seminal filaments are comparable to flagellate cells. They 

 are usually composed of three portions, head, middle piece, and 

 contractile filament. 



3. The seminal filament is developed out of a single cell, the 

 spermatid ; the head, and probably also the middle piece, from the 

 nucleus ; the contractile filament from the protoplasm. 



4. The egg-cell consists of egg-plasm and yolk-particles, which are 

 reserve material (deutoplasm), imbedded in it. 



5. The quantity and distribution of the deutoplasm in the egg-cell 

 is subject to great variation, and exercises the greatest influence on 

 the cour.se of the first processes of de\'elopment. 



(a) The deutoplasm is small in amount, and uniformly dis- 



tributed in the egg-plasm. 



(b) The deutoplasm is present in greater quantity, and, in 



consequence of unequal distribution, is more densely 

 accumulated either at one pole of the egg or in its middle. 

 (Polar and central deutoplasm.) 



(c) 111 eggs with polar deutoplasm (eggs with polar differentia- 



tion) the pole with more abundant deutoplasmic contents 

 is designated as the vegetative, the opposite one as the 

 animal pole. 



(d) In the case of eggs with polar differentiation, the more 



abundant protoplasm of the animal pole may be sharply 

 differentiated as germ-disc (formative yolk) from the 

 portion which is richer in deutoplasm (nutritive yolk). 

 The developmental processes take place only in the 

 formative yolk, while the nutritive yolk remains on the 

 whole passive. 



