GASTRULATION 



799 



spheres, which are at the periphery, divide, each one into two, making 

 eight epiblastic and four hypoblastic spheres. When this has occurred, 

 the epiblastic spheres are smaller and clearer than the hypoblastic 

 spheres. The four hypoblastic spheres now divide" into eight. The 

 epiblastic spheres then divide into sixteen, the hypoblastic spheres in 

 turn divide, and this goes on until the process of segmentation is com- 

 pleted. In the rabbit, this occurs usually about seventy hours after 

 impregnation. As segmentation progresses, the epiblastic cells extend 

 over the hypoblastic cells, and become irregularly-polygonal in form. 

 The hypoblastic cells occupy the central portion of the ovum. At first 

 there is a circular space on the ovum where the epiblastic cells do not 

 cover the cells of the hypoblast (see Fig. 219, A); but this soon becomes 

 closed by an extension of the cells of the epiblast (see Fig. 219, B). The 

 hypoblastic cells, at the close of segmentation, are slightly larger than 

 the cells of the epiblast and are darker and more rounded. The ovum 

 now consists of a solid mass of cells and is called the morula, on account 

 of its fancied resemblance to a mulberry. The cells of which it is com- 

 posed are called collectively blastodermic cells. The ovum probably is 

 in this condition when it passes from the Fallopian tube into the uterus. 



Most of the phenomena of segmentation have been observed in the 

 lower forms of animals ; but there can be no doubt that analogous pro- 

 cesses take place in the human ovum. In the rabbit, forty-five and a 

 half hours after copulation, Wiel observed an ovum with sixteen seg- 

 mentations, situated in the lower third of the Fallopian tube. He 

 observed an ovum, ninety-four hours after copulation, with a delicate 

 mosaic appearance, presenting a small rounded eminence on its surface. 

 It is impossible to say how long the process of segmentation continues 

 in the human ovum. It is stated that it is completed in rabbits in a few 

 days, and in dogs, that it occupies more than eight days (Hermann). 



Gastrulation. After segmentation has been completed, a cavity 

 filled with liquid appears between the hypoblastic and epiblastic cells, 

 except at that portion which has last been covered by the epiblast. Here 

 the cells of the hypoblast are in contact with the epiblast. The liquid in 

 the interior of the ovum gradually increases in quantity, the ovum be- 

 comes enlarged to the diamater of ^ to ^V f an mcn (-5 to J milli- 

 meter), and is now called the blastula, or blastodermic vesicle. The 

 epiblastic cells surround the blastodermic vesicle completely, forming a 

 single layer over the greater portion ; and the hypoblastic cells form a 

 lenticular mass attached to the smaller portion of the inner surface of 

 the layer of epiblastic cells (see Fig. 219, C and D\ There then occurs 

 a gradual invagination of the hypoblastic cells, forming a double-walled 

 sac, called the gastrula. This is the primitive gut, or archenteron. The 



