12 



NORMAL HISTOLOGY. 



Head 



-* 



Tail 



the nucleus. Towards the periphery of the cell they are nearly wanting, a 

 narrow zone of almost homogeneous cytoplasm lying immediately beneath 

 the delicate vitelline membrane. The liberated ovum is surrounded by a 

 protecting membrane, the zona pellucida, which some- 

 times exhibits a faint radial striation. This envelope 

 must not be confounded with the vitelline membrane, 

 since it is not strictly a part of the ovum, but a product 

 of the surrounding epithelial cells lining the little sac, the 

 Graafian follicle, enclosing the egg while within the ovary. 

 The large eccentric spherical nucleus, the germinal vesicle, 

 is about 37 //' in diameter and surrounded by a distinct 

 nuclear membrane. Within the germinal vesicle are 

 found the usual constituents of the nucleus, including 

 the all-important chromatin fibrils, nuclear matrix and 

 nucleolus. The latter, the germinal spot, is distinct and 

 measures about 5 // in diameter. 



The spermatozoon, the male germ-cell, is produced 

 by the specialization of epithelial cells lining the semi- 

 niferous tubules within the testis. The human sperma- 

 tozoon consists of three chief parts the ovoid head, 

 middle-piece, which includes the slightly constricted neck 

 and the connecting piece, and the attenuated and greatly 

 extended tail. Although the entire length of the sper- 

 matic element is about 50 //, the head measures only 

 about 5 IJL ; the male germ-cell, therefore, is much smaller 

 than the ovum. The head and the neck are the most 

 important parts, since they contain respectively the chro- 

 matin and the centrosome of the cells, the spermatids > 

 from which the spermatozoa are directly derived. 



The centrosome is represented by two minute spher- 

 ical bodies, the neck-granules, which lie in the neck 

 immediately beneath the head and at the anterior ex- 

 tremity of the connecting piece. The axial fibre extends 

 throughout the spermatozoon from the neck to the tip 

 of the tail, ending as an attenuated thread, the terminal 

 filament. The tail corresponds to a flagellum and serves the purpose of 

 propulsion alone, taking no part in the important changes within the ovum 

 incident to fertilization, during which the head and middle-piece enter the 

 substance of the egg. 



Immediately following the construction of a new nucleus from the chro- 

 matin contributed by the two parental germ-cells, the fertilized ovum enters 

 upon a cycle of repeated division. As the result of this process, known as 

 segmentation, in which the new cells arise by mitotic division, a spherical 

 mass of young cells, the morula, is produced. This mass, at first solid, 

 soon acquires a central cavity filled with fluid and is converted into a hollow 

 sphere, known as the blastodermic vesicle. The wall of this sac consists of 

 a single layer of cells, except at one place where a small mass of cells is 

 attached to the inner surface. The outer or covering layer of cells is the 

 trophoblast ; the group of cells attached to the inner surface pf the tropho- 

 blast is the inner cell-mass. Corresponding to the position of the latter, 



Terminal 

 filament 



FIG. 9. Diagram of 

 human spermatozoon ; 

 a, neck-granules, rep- 

 resenting the centro- 

 some; 6, axial fibre. 

 X 1800. (Meves). 



1 The sizes of microscopic objects are usually expressed in thousandths of a 

 millimeter, represented by the letter/*; i fi (micron) = .001 mm. 



