CHAP. XLI.] PRIMITIVE STREAK. 585 



but whether its contents become mixed up with the substance of 

 the yolk, or new cell-formations primarily result from it, has not 

 yet been conclusively determined. 



By the aggregation of cells upon the surface of the yolk, a sort 

 of membrane, composed of pentagonal cells with nuclei is formed 

 within the zona pellucida, or yolk -membrane. This is called the 

 blastodermic vesicle, and in its mode of formation nearly corresponds 

 to the germinal membrane of the bird's egg. 



The chorion, or outermost membrane investing the ovum, is 

 formed by the gradual coalescence of the zona pellucida, and the 

 layer of albuminous material with which it is covered. The chorion 

 is at first smooth, but villi are subsequently developed all over its 

 surface. Beneath the chorion is the blastodermic vesicle, which 

 gradually increases in thickness by the growth of new cells upon 

 its internal surface. 



In consequence of the great difficulty experienced in procuring 

 human ova at a very early period in a perfectly normal state, and 

 in sufficient number, physiologists have been compelled to make 

 direct observations upon the lower animals, and to assume that a 

 series of changes precisely similar takes place in the human ovum, 

 an inference which has been of late years fully justified by direct 

 observation. In discussing this part of our subject, we shall, 

 therefore, infer that the early embryonic changes occur in a similar 

 order, and are of the same essential character in all mammalia, 

 except, of course, with reference to the precise period at which they 

 take place, which necessarily differs in various animals according 

 to the duration of pregnancy. For much that we know of the 

 development of the mammalian ovum, we are indebted to the beau- 

 tiful researches of Bischoff upon the ova of the dog, rabbit, 

 guinea-pig, and deer. 



Primitive Streak. At a period varying from the twelfth to the 

 sixteenth day, the dog's ovum assumes a more oval form. It is 

 about three lines in length, and about a line and a half in its short 

 diameter. Its external surface is as yet perfectly smooth, for none 

 of the tufts or villosities of the chorion are developed. The central 

 clear space (area pellucida or germinitiva) is seen to be surrounded 

 by a darker circle, which eventually becomes the vascular area. 



In the centre of the clear space a line is soon observed. This 

 is the first trace of the embryo, the primitive streak, which appears 

 in the form of a straight white line, or very shallow groove. It lies 

 across the short axis of the ovum, and therefore occupies a position 

 precisely similar to the embryo chick. 



VOL. II. Q Q 



