18 GENERAL EMBRYOLOGY 



organisms are in the form of single cells, i.e., gametes. And 

 thus it comes about that in the many-celled animals, reproduc- 

 tion and syngamy are so uniformly associated, and while these 

 processes may not have been related primarily, in some in- 

 stances are not even at present, yet now they have come to 

 be so related in the vast majority of Metazoa, that fertilization 

 actually appears as the first and most important step in the 

 whole chain of reproductive events. 



The actual processes involved in the formation, from the 

 zygote, of the mature Metazoan individual are extremely 

 complicated and diverse, but they are for the most part reducible 

 to three fundamental general processes. We must leave aside, 

 for the present, the causal or directive processes which, though 

 probably the essentials of development, are still obscure and 

 little known. The grosser external phenomena of development 

 are, essentially, growth, cell division, and differentiation. The 

 living germ is contained within the limits of a single cell, often 

 of minute dimensions and only slightly differentiated visibly; 

 the mature organism consists of an enormous number of cells, 

 comprising a considerable mass, and exhibiting various degrees 

 of differentiation in diverse directions. The transition from 

 one of these states to the other is a gradual process, proceeding 

 by minute steps; yet it is convenient to consider the whole life 

 history of an organism as a succession of phases, each with 

 some chief characteristic. 



First, complex processes occur within the gametes or germ 

 cells themselves, concerning chiefly their nuclei, as a result of 

 which they come to have a constitution quite unlike that of the 

 somatic nuclei. These preliminary events we group under the 

 term gametogenesis, or oogenesis and spermatogenesis in the ova 

 and sperm cells respectively. Then normally follows fertiliza- 

 tion or syngamy, the fusion of the two gametes, derived from two 

 different organisms, into a single cell which is the "new" 

 organism. Through fertilization a typical nucleus is recon- 

 stituted in the zygote, and there follows a period of rapid cell 

 multiplication which is called the period of segmentation or 

 cleavage. During cleavage are formed the cellular elements 



