STRUCTURE OF THE BLASTODERM. 



86 3 



Variation of Lines of Cleavage. According to the observations of Pfliiger, the ova of the 

 frog can be made to undergo cleavage in very different directions, according to the angle between 

 the axis of the egg and the line of gravitation. This of course we can alter as we please, by 

 placing the eggs at any angle to the line of gravitation. By the axis of the ovum is meant a 

 line connecting the centre of the black surface and the middle of the white part, which, in the 

 fertilised ovum, is always vertical. In such cases of abnormal cleavage the deposition of the 

 organs takes place from other constituents of the egg than those from which they are formed 

 under normal conditions. Under normal circumstances, according to Roux, the first line of 

 cleavage in the frog is in the same direction as the central nervous system. The second inter- 

 sects the first at a right angle, so as to divide the mass of ovum into two unequal parts, the 

 larger of which forms the anterior part of the embryo. 



Blastoderm. During this time the ovum is enlarging by absorption of fluid 

 into its interior. All the cells, from mutual pressure against each other, become 

 polyhedral, and are so arranged as to form a cellular envelope or bladder, the 

 blastoderm or germinal membrane, which lies on the internal surface of the 

 vitelline membrane (De Graaf, v. Baer, Bischoff, Coste). A small part of the cells 

 not used in the formation of the blastoderm is found on some part of the latter. 

 [In the ovum of the bird, where there is only partial segmentation, the blastoderm 

 is a small round body resting on the surface of the yelk, under the vitelline 

 membrane, so that it does not completely surround the yelk, or a hollow cavity, as 



Fig. 660. Fig. 661. 



Fig. 660. Blastodermic vesicle of a rabbit, ect, ectoderm, or outer layer of cells ; ent, inner 

 layer of cells. Fig. 661. Pr, primitive streak ; B, medullary groove ; U, first proto- 

 vertebra. 



in mammals. In mammals, this cavity is called the segmentation cavity.] The 

 hollow sphere, composed of cells, is called the blastodermic vesicle by Reichert 

 (fig. 660), and in the human embryo it is formed at the 10th to 12th day, in the 

 rabbit at the 4th, the guinea-pig at the 3 J, the cat 7th, dog 11th, fox 14th, 

 ruminantia at the 10th to 12th day, and the deer at the 60th day. 



When the blastoderm grows to 2 mm. (rabbit), whereby the vitelline membrane 

 is distended to a very thin delicate membrane, then at one part of it there appears 

 the germinal area, the area germinativa, or the embryonal shield (Coste, Kdlliker), 

 as a round white spot, in which the blastoderm, owing to the proliferation of its 

 cells, becomes double. The upper layer is called the ectoderm or epiblast, and in 

 some animals it consists of several layers of cells, while the lower layer is the 

 endoderm or hypoblast. The hypoblast continues to grow at its edges, so that it 

 ultimately forms a completely closed sac, on which the epiblast is applied concen- 

 trically. The embryonal area soon becomes more pear-shaped, and afterwards 

 biscuit-shaped. At the same time the surface of the zona pellucida develops 

 numerous small, hollow, structureless villi, and is called the primitive chorion. 



At the posterior part, or narrow end, of the embryonic shield, the primitive 

 streak (fig. 661, I, Pr) appears at first as an elongated opaque circular thickening, 

 and later as a longer streak or groove, the primitive groove. [The opacity is due 



