DEVELOPMENT OF THE EMBRYO 449 



exception of the middle area above the nucleus of Pander. This area 

 continues to remain free from nuclei ; in fact, what is later to be known 

 as the germ wall is partly composed of the nucleated rim of the peri- 

 blast. 



The blastoderm, which is rather circular, extends radially., both on 

 account of the growth of its own cells, and by the addition of cells from 

 the marginal periblast. The original region of the blastodisc becomes 

 thinner and transparent. It is then called the area pellucida. The cir- 

 cular margin which is derived from the periblast is called the area opaca. 

 The ring-like periblast keeps on growing, while additional nuclei are 

 formed peripherally. At the same time, the periblast is contributing 

 cells to the blastoderm also, so that it steadily increases in diameter. 

 The inner nucleated margin of the periblast, which later becomes cellu- 

 lar, contributes to the later extra-embryonic tissues and is called the 

 germ wall. The cells of the blastoderm later extend peripherally so that 

 they overlap the inner margin of the germ wall, to form a narrow region 

 transitional between pellucid and opaque areas. 



It should be noted here that the lower surface of the periblast is 

 directly continuous with the yolk mass, and peripherally it is continuous 

 with a very thin superficial tissue of protoplasm. This latter is also 

 often referred to as a part of the germ wall. 



As soon as the blastoderm has become thinned out as mentioned 

 above, the blastula stage is completed. 



It is well at this point partially to summarize the development 

 through the morula and blastula stage before taking up gastrulation. 



THE MORULA STAGE 



While text-books usually speak of an "end" to the segmentation 

 process, it must not be supposed that the cells of the embryo stop divid- 

 ing. The whole process is continuous, and the word "end" here means 

 only that the general process of cell-division is now "general" no longer, 

 but that differentiation begins. The ending of the segmentation stage 

 means only that one can from this period on find a grouping or aggre- 

 gation of cells which are not all alike. 



In eggs in which there is but little yolk, the segmentation results 

 in a rounded, closely packed mass of embryonic cells (blastomeres) 

 called a morula. This name has been given such a cell mass because it 

 resembles a mulberry. This morula stage, in eggs with little yolk, cor- 

 responds to the stage in the "end" of segmentation in the chick embryo. 

 At this time the embryo is a simple disc-shaped mass of cells, several 

 layers in thickness. This whole mass is the blastoderm. It lies closely 

 applied to the yolk. 



The cells in the center of the blastoderm are smaller and quite 

 clearly defined, while the surrounding or peripheral cells are flattened, 

 larger, and in more intimate contact with the yolk beneath. 



