24 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



hypoblast (Fig. 14). From these three layers of cells the embryo is 

 developed. 



Leaving at this point the changes which occur in the egg of the bird, 

 we have now to follow the analogous changes in the mammalian ovum. 



We have already seen that in the mammalian ovum one of the first 

 evidences of impregnation is the division of the protoplasm of the ovum 

 progressively into smaller and smaller segmentation spheres, until the 

 cell-membrane becomes filled with an immense number of minute masses 

 of protoplasm. The general character of this process in its earlier stages 

 is probably identical in all the mammalia. The ovum of the rabbit has 

 been most studied, and the sketch here given is based mainly on Balfour's 

 summary of the early stages of development in the rabbit's ovum. 



The ovum first divides into two nearly equal spheres, of which one 



BD 



BD, BD. MC. 



FIG. 14. SECTION OF A BLASTODERM OF CHICK, AT RIGHT ANGLES TO THE 

 LONG Axis OF THE EMBRYO, AFTER EIGHT HOURS' INCUBATION, ABOUT 

 MIDWAY BETWEEN FRONT AND HIND ENDS. (Foster and Balfvur.) 



A, epiblast; B, mesoblast; C, hypoblast: PR, primitive groove; F, fold in the blastoderm produced 

 accidentally ; MC. mesoblast-cell, the line points to one of the peripheral mesoblast-cells lying between 

 epiblast and hypoblast; BD, formative cells. 



The section shows: (1) the thickening of the mesoblast under the primitive groove, PR, even when it 

 is hardly present at the sides of the groove : (2) the hypoblast, C, early formed as a single layer of spindle- 

 shaped cells ; (3) the so-called segmentation cavity, in which coagulated albumen is present. On the 

 floor of this are the large formative cells, BD. 



is slightly larger and more transparent than the other. The larger sphere 

 and its products will be spoken of as the epiblastic spheres ; the smaller 

 one and its products as the hypoblastic spheres. Both these original 

 spheres soon divide into two, and each of these into two more, thus 

 making eight. At first these spheres are spherical, and arranged in two 

 layers formed of four epiblastic and four hypoblastic spheres. Soon r 

 however, one of the hypoblastic spheres passes into the centre, and the 

 whole ovum becomes spherical again. 



In the next stage each of the four epiblastic spheres divides into 

 two, followed b} r the division of the hypoblastic spheres into two. The 

 ovum is then made up of sixteen different spheres, nearly of the same 

 size. Of the eight hypoblastic spheres four soon pass to the centre, and are 

 surrounded by the eight epiblastic spheres, arranged in the form of a cup. 

 Division of both sets of spheres now continues, the epiblastic layer con- 



