CHAPTER VIII. 

 THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



The segmentation of the ovum and the formation of the blastodermic vesicle 

 have not been observed in man. For these stages it is necessary, therefore, to 

 depend upon the lower Mammals. In those Mammals in which the processes 

 have been observed, the segmentation of the ovum produces a solid mass of 

 cells known as the morula (Fig. 88; compare with Fig. 33). The superficial 

 cells of the morula then become differentiated from those m the interior. The 

 result is a solid sphere composed of a central mass of polyhedral cells and an 

 enveloping layer of somewhat flattened cells (Fig. 88; compare with Fig. 33). 

 The cells of the enveloping layer become still more differentiated from those of 

 the central mass, and the sphere continues to increase in size owing to the pro- 

 liferation of both kinds of cells. The next step in development is the formation 

 of a cavity within the sphere. Among Invertebrates, where but little yolk is 

 present and where no 'distinct differentiation of the superficial cells occurs, the 

 central cells are displaced, or pushed toward the periphery, so that the morula is 

 changed into a hollow sphere the blastula the wall of which is composed of a 

 single layer of cells (p. 50). Among Mammals, however, instead of a displace- 

 ment of the central cells, there appear within the cells vacuoles which continue 

 to enlarge and finally become confluent, thus forming a cavity which occupies 

 the greater part of the interior of the sphere. There remain then, after the 

 vacuolization, the enveloping cells, or trophoderm, and a few of the central cells 

 which are attached to the trophoderm over a small area and constitute the 

 inner cell mass (Fig. 88). The latter is the anlage of the embryonic body. 

 As stated on page 52, the cavity of the sphere in Mammals is not homologous 

 with the cavity of the blastula in the lower forms, but the vacuolization of the 

 cells probably represents a belated and abortive attempt at yolk formation. 



Following the formation of the yolk cavity, those cells of the inner cell mass 

 which border it become differentiated, proliferate and gradually spread out in a 

 single layer that finally forms a complete lining for the cavity. The cells of this 

 layer constitute the primitive entoderm (Fig. 88) . In the meantime some of the 

 cells of the inner cell mass which lie between the differentiating entoderm and 

 the trophoderm undergo a process of vacuolization, leaving only a single layer 

 closely applied to the entoderm. This layer is the embryonic ectoderm, and the 

 newly formed cavity between it and the trophoderm is the amniotic cavity 



137 



