THE OVUM. 65 



thicker on one side than on the other, and there appears in the 

 thickened mass a cavity, which gradually becomes more dis- 

 tended and is filled with an albuminous fluid. As the cavity 

 enlarges, the ovum with several layers of cells around it forms 

 a prominence projecting into it. The whole structure with, its 

 tunic is known as the Graafian follicle. The follicle cells are 

 known as the membrana granulosa, and the projection, in 

 which the ovum lies, as the discus or cumulus proligerus. 

 The cells of the discus in immediate contiguity to the ovum 

 usually form a more or less specialized layer and are somewhat 

 more columnar than the adjoining cells. 



THE SPERMATOZOON. 



Although there is no doubt that the spermatozoon in most 

 instances plays as important a part as the ovum in influencing 

 the characters of the organism which is evolved from the 

 coalesced product of the ovum and spermatozoon, yet the 

 actual form of the spermatozoon has not, like the form of the 

 ovum, a secondary influence on the early phases of development. 

 A comparative history of the spermatozoon is therefore of less 

 importance for my purpose than that of the ovum ; and I shall 

 confine myself to a few remarks on its general structure, and 

 mode of growth. The primary origin of the male germinal cells, 

 and their relation to the sperm- forming cells, is dealt with in 

 the second part of the treatise. 



Although the minute size of most spermatozoa places great 

 difficulties in the way of a satisfactory investigation of them, yet 

 there can be but little doubt that they always have the value of 

 cells. In the vast majority of instances the spermatic cell or 

 spermatozoon is composed of (i) a spherical or oval portion 

 known as the head, formed of a nucleus enveloped in an 

 extremely delicate layer of protoplasm, and (2) of a motile 

 protoplasmic flagellum known as the tail; which together with 

 the investing layer of the head forms the body of the cell. 



As might be anticipated, the proportion, size, and relations 



of the parts of the spermatozoon are subject to great variations. 



The head is often extremely elongated ; and it is in many cases 



rather on theoretical grounds, than as a result of actual observa- 



B. II. 5 



