Chap. VI.] GENERAL EMBRYOLOGY. 87 



is no distinction between ovary and testis. Ere long, however, 

 differentiation sets in, and different lines of development are 

 followed in the two sexes. 



In the male the testis becomes a mass of minute tubes, the 

 seminal tubules, twisted and convoluted in all directions, and 

 lined with large cells, probably derived from the original 

 germinal cells, which are destined to give rise to spermatozoa. 

 They are termed sperm cells. From these a number of 

 smaller cells arise by a process like budding. It is these smaller 

 cells (spermatoUasts) which are converted into spermatozoa. 

 The tubules in which they are developed open into the vas 

 deferens, by which the ripe spermatozoa are carried out of the 

 body. 



In the ovary of the female there is no development of tubules. 

 Certain larg germinal cells are developed at the expense of 

 other cells. These are the ova. The cells adjoining each ovum 

 arrange themselves round it so as to form a special layer which 

 forms the lining of a sort of bag, the ovisac, in which the ovum 

 lies. When the ovum is ripe the ovisac bursts, and the egg is 

 set free into the peritoneal cavity, or passes at once into the 

 oviduct, and so out of the body. 



The very young ovum, before it is mature, consists of a minute 

 spherical mass of granular protoplasm. Within it is a specialised 

 portion differing from the rest, and called the germinal vesicle, 

 and within this again may be seen one or several germinal spots. 

 The germinal vesicle is invested by a delicate membrane, and 

 stretching from this membrane to the germinal spot or spots 

 is a network of delicate fibres called the reticulum. The ovum is 

 a simple cell. The germinal vesicle may represent the nucleus 

 of the cell, and each germinal spot a nucleolus. 



Such is the immature ovum. From it the mature ovum 

 differs (1) in the possession of an external membrane; and (2) 

 in the development within it of a greater or less quantity of 

 food-yolk in the form of minute masses known as yolk-spheres. 

 Both the amount and the mode of distribution of the food-yolk 

 are matters of considerable importance. The amount differs 

 markedly in our three types. The minute ovum of the rabbit 



