SEGMENTATION. 



21 



Polar bodies 



Oolemma 



Polar bodies 



Oolemma 



SEGMENTATION. 



Immediately after its formation the zygote is separated, by a series of consecu- 

 tive mitotic divisions, into a large number of cells which are grouped together 

 in the form of a solid spherical mass, called a morula on account of the mulberry- 

 like appearance of its surface. This period of division 

 is called the period of segmentation (Figs. 24-2*7). 



The segmentation divisions are of the homotype 

 form, and there is evidence which tends to the con- 

 clusion that the earliest divisions, by which the zygote 

 is divided first into two and then into four parts, are 

 quantitatively and qualitatively equal. After a time, 

 however, the divisions result in the formation of cells 

 of different sizes and different capabilities, definite 

 and circumscribed functions being allocated to certain 

 groups of cells and their descendants. It is probable FIG. 24.-SEGMENT OF ZYGOTE 

 that at this time cells are set apart which are the pro- 

 genitors of the germ cells of the next generation, and 

 which therefore retain all the capabilities of their 

 ancestors. These cells are the means by which the 

 species is reproduced and the hereditary tendencies 

 are transmitted from generation to generation. At the 

 same time other cells are set apart for the production 

 of the tissues and organs of the individual which will 

 be produced from the zygote, and in which the germ 

 cells and their descendants will be lodged and protected 

 till they attain their maturity. 



After the morula is established one of the first 

 definite changes which occurs in its constitution is 

 the differentiation of its cells into an outer layer 

 and an inner mass (Fig. 26). 



In the human subject, as in many other mammals, 

 the cells of the outer layer constitute the trophoblast 

 or trophoblastic ectoderm, which plays a most im- 

 portant part in the nutrition of the embryo and fcetus. 

 It enters into the formation of the chorion, or outer- 

 most envelope of the growing zygote, which is sub- 



sequently differentiated into a placental and a non- FM . 26 ._ gMMENTATIONOpZY60T] , 

 placental portion and which serves, in the nrst in- 

 stance, both as a protective and a nutritive covering. 



In many mammals the cells of the inner mass 

 soon separate into two main groups, the ecto-mesoderm 

 and the entoderm ; but it appears probable that, in the 

 human subject, they differentiate into three groups, 

 ecto-mesoderm, primary mesoderm, and entoderm. 



In the majority of mammals, immediately before or 

 as the differentiation of the inner mass occurs, a cavity 

 appears in the zygote. As soon as the cavity appears 

 the morula is converted into a blastula and the cavity 

 enlarges until it separates the inner mass from the 

 outer layer, except at one pole of the zygote, where the 

 inner mass and the outer layer remain in contact. 

 The cavity is called the segmentation cavity. It 

 would appear, however, from the evidence at present 

 available, that this primitive cavity never exists in the human subject, for as the 

 main part of the inner mass separates from the outer layer the cells of the primary 

 mesoderm segment of the inner mass proliferate rapidly and form a jelly-like tissue 

 which completely fills the space which would otherwise become the segmentation 

 cavity. At the same time the ecto-mesodermal and entodermal segments of the 



Morula Stage. 



Trophoblast 

 Ecto-mesoderm 

 Ectoderm * 



Entoderm' 

 Primary mesodertn 



FIG. 27. DIFFERENTIATION OF 

 ZYGOTE AND CELLS (Hypothetical). 



