INTRODUCTIOJi 3 



features in the mode of development are the result of 

 common descent. The earliest stages of development 

 in the Metazoa can readily be reduced to a uniform plan 

 characterized by the appearance of the blastula- and gastrula' 

 stages at the end of cleavage. One is justified in the as- 

 sumption that in these two stages there exists a repetition 

 of ancestral forms which are common to all the Metazoa. 



In the first stages of development of the Metazoa the 

 existence of a chief or 'primary axis can be recognized, the 

 ends of which are distinguished as the animal pole and the 

 vegetative pole, because in the differentiation of the two 

 primary germ-layers, which soon follows, the layer arising 

 in the vicinity of the animal pole (ectoderm) presides over 

 the animal functions (sense perception, locomotion), while 

 the germ-layer at the opposite pole (entoderm) is mainly de- 

 voted to the functions of vegetation (e.g., nutrition). The 

 Metozoa accordingly at first show a monaxial, heteropolar 

 structure. Frequently the chief axis can be recognized in 

 the egg-cell of the Metazoa before the beginning of 

 development, since the germinative vesicle (nucleus of 

 the egg-cell) and a dense accumulation of protoplasm are 

 situated near the animal pole, whereas in the region of the 

 vegetative half of the egg a great accumulation of yolk 

 particles can be recognized. The animal pole, furthermore, 

 is characterized by being the place at which the expulsion 

 of the polar globules takes place before fertilization. 



The process of the cleavage of the egg, by which, after 

 fertilization has taken place, the embryonic development is 

 initiated, is essentially an ever-progressing division of the 

 egg, which takes place according to fixed laws, and by which 

 the egg is divided into a number of cells (cleavage spheres, 

 blastomeres), whicli at first are still undifferentiated. Ac- 

 cording to the direction which the planes of cleavage occupy 

 in this process, we distinguish meridional and equatorial 

 furrows, the former coinciding with the chief axis, the latter 

 being perpendicular to it. In this manner there arise 

 blastomeres that are at first spherical, but in later stages 

 more or less pyramidal in form, and which are arranged 

 radially about a point occupying the centre of the egg. By 



