i6i2 A MANUAL OF ANATOMY 



a great number of cells, which are called blastomeres, or segmenta- 

 tion-cells. At the first division, which affects primarily the seg- 

 mentation nucleus, the ovum is cleft into two cells, which lie close 

 together, the opposed surfaces being flattened. At the second 

 division each of these cells is cleft into two, so that four blastomeres 

 now occupy the interior of the ovum. Each of these, in turn, 

 divides into two, thus giving rise to eight blastomeres. This process 



First Division Final Division 



Fig. 666. — Segmentation of Ovum. 



of cell-division goes on, sixteen blastomeres being formed, succeeded 

 by thirty-two, and so on. Finally, the ovum, originally simple, 

 becomes transformed into a heap of nucleated blastomeres, or seg- 

 mentation-cells, the superficial cells being clear, whilst the more 

 deeply placed cells are granular. These constitute a solid, spherical, 

 mulberry-like mass, called the morula, and this stage is hence known 

 as the morula-stage. 



Formation of the Blastodermic Vesicle and Blastoderm. 



After the morula-stage has been completed, fluid accumulates 

 within the morula and a cavity is formed, called the segmentation- 

 cavity or blastocoele. The ovum is now a vesicle, and is known 

 variously as the blastula, or blastodermic vesicle. During the 

 formation of the segmentation-cavity the blastomeres or cells of 

 the morula become arranged in two groups — outer and inner. The 

 cells of the outer group, which are clear, spread out and form a 

 membrane, which is known as the enveloping layer, or irophoblast. 

 This layer represents the primitive ectoderm, and it encloses (i) the 

 inner group of cells, and (2) the segmentation-cavity. That portion 

 of the enveloping layer which covers the inner group of cells at the 

 upper pole of the blastodermic vesicle is known as Rauher's covering 

 layer. The primitive ectoderm (enveloping layer), being extra- 

 embryonic, gives rise to the foetal part of the placenta and the 

 foetal membranes. 



The cells of the inner group, which are granular and spherical, 

 and which originally filled the morula, become gradually collected 

 into a round heap, known as the inner cell-mass. This mass is 

 situated at the upper, embryonic, or germinal pole of the blasto- 



