52 CHORD AT A. 



funnel-shaped form with the narrow end in contact with the egg- 

 membrane. These large cells, which have a regular arrangement in 

 the epithelium, are probably in some way connected with the nutri- 

 tion. They have only been noticed in large-yolked ova. Many 

 observers have described prolongations of the follicle cells through 

 the pores of the zona radiata in Aves, Reptilia and Teleostei. 



The most remarkable modification of the follicle is that which is 

 found in Mammalia. At first the follicle is similar to that of other 

 Vertebrata, and is formed of flat cells derived from the germinal cells 

 adjoining the ovum. These cells next become columnar and then 

 one or two layers deep. Later they become thicker on one side than 

 on the other, and there appears in the thickened mass a cavity, which 

 gradually becomes more distended 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 ceils, 

 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 (1) 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 observation, that 



