EXPLANATION OF PLATES. 





PLATE I. 



FIG. 



1. Spermatozoa of Man; A, viewed on the surface; B, viewed edgewise (g 958). 



2. Vesicles of evolution from the seminal fluid of the Dog ; A, B, c, single vesicles of dif- 



ferent sizes ; D, single vesicle within its parent-cell ; E, parent-cell inclosing seven 

 vesicles of evolution ($ 959). 



3. Development of Spermatozoa within the vesicles of evolution ; A, B, vesicles containing 



spermatozoa in process of formation ; c, D, spermatozoa escaping from the vesicles 

 (2 959). 



[The three preceding figures are after Wagner and Leuckardt ("Cyclop, of Anatomy 

 and Physiology," Art. "Semen").] 



4. Thin slice of the ovarium of a Sow three weeks old, showing the Graafian vesicles or 



ovisacs imbedded in a fibro-cellular stroma. The ovisacs are filled with cells, in 

 the midst of which one large one may be specially distinguished ; this, which is 

 the germinal vesicle, is surrounded by minute granules, which constitute the first 

 indication of the yelk (| 963). 



5. Ovum of a Rabbit, showing the vitelline mass almost entirely converted into distinct 



cells, of which those at the surface are pressed against each other and against the 

 zona pellucida, so as to assume an hexagonal form. The dark portion consists of 

 a mass of vitelline spheres, which has not undergone this conversion ( 996). 



6. Ovum of the Rabbit, seven days after impregnation, viewed on a black ground. The 



outer membrane is the chorion, on which are seen incipient villosities. Within this 

 is the blastodermic vesicle, at the summit of which is the projection formed by the 

 area aerminativa ; and from this the mucous layer of the germinal membrane is 

 seen to extend over about one-third of the surface of the contained yelk (g 996). 



7. Portion of the germinal membrane, taken from the area aerminativa, to show the two 



layers of which it is composed ; the serous, or animal layer, is turned back, so as 

 to show the mucous, or vegetative, layer in situ. In the latter is seen the primitive 

 trace ( 996). 



8. Portion of the serous layer of the germinal membrane, highly magnified ; showing that 



it is made up of nucleated cells, united by intercellular substance, and filled with 

 minute molecules ( 996). 



9. Portion of the mucous layer of the germinal membrane, highly magnified ; showing that 



it is made up of cells whose borders are more distinct and more closely applied to 

 each other than those of the serous layer, and whose contents are more transparent 

 (8 996). 



[The six preceding figures are after Bischoff ("Entwickelungsgeschichte der Siiuge- 

 thiere," &c. (1842); "des Kanincheneies" (1842); "des Hunde-eies" (1845).] 



