CHAPTEE I 



SEGMENTATION, GASTEULATION, AND THE 

 FOEMATION OF THE GEBM LAYEES 



The Vertebrate begins its individual existence in the form of a single 

 cell, the Zygote or fertilized egg, which in turn originates in the 

 process of fertilization by the fusion or conjugation of two gametes. 1 

 Of these the microgamete or spermatozoon, derived from the male 

 parent, is of relatively insignificant bulk as compared with the 

 niacrogamete or unfertilized egg. As a consequence the more 

 obvious features of the Zygote, such as shape, size, and so on, are 

 simply taken over from the macrogamete — in other words, they are 

 of maternal origin. Such maternal features may remain obvious for 

 some time during early stages of development, so long in fact as the 

 maternal protoplasm remains predominant in bulk as compared with 

 that elaborated under the control of the Zygote nucleus, but it seems 

 unnecessary to assume that this fact has the important bearing upon 

 questions connected with Heredity which has been claimed for it by 

 some workers on Invertebrates. 



The Zygote is a typical cell, composed, so far as its living substance 

 is concerned, of cytoplasm and nucleus, the cytoplasm containing a 

 lesser or greater amount of food-material or yolk. In shape it is in 

 the vast majority of cases approximately spherical. In the Myxinoids 

 it is elongated, almost sausage-shaped, and in a certain number of 

 cases, for example Amia, its shape is literally " oval." 



The macrogamete — and therefore the Zygote — differs much in 

 size in different Vertebrates, ranging from about - 1 mm. in diameter 

 in Amphioxus to as much as 85 mm., or more, in the. case of the 

 African Ostrich. In some of the Sharks the size of the Zygote is 

 also very great and this was doubtless the case too, with that of such 

 extinct birds as Aepyornis. 2 Such relatively huge Zygotes are of 



1 A general account of the processes of gametogenesis and fertilization has already 

 been given in Vol. I. and they are not further dealt with in this volume. 



2 Assuming that the Zygote of Aepyornis bore the same ratio in size, to its protective 

 envelopes as does that of the Ostrich it would measure about 160 mm. in diameter. 



VOL. II B 



