DEVELOPMENT 191 



senses in which these terms are used will be understood from the 

 above summaries. But these various phases are often not well 

 separated from each other. 



At all phases of the life-history the organism is a perfectly 

 specific thing. If we have (from ordinary observ^ation) sufficient 

 knowledge of the life-histories of a group of animals the latter 

 can always be identified as the separate species at all stages in 

 their life-histories. Thus the eggs of British birds are all easily 

 identifiable as species even when we do not know what the parent 

 was. The eggs and larvae of the British Teleostean fishes are 

 all as clearly separable from each other and identifiable as are the 

 fully-grown parents. Any life-history whatever is a specific 

 career and is different from all others. The study of life-histories 

 involves almost interminable detail and is the subject of the 

 systematic works on Zoology — it is very incomplete and there are 

 comparatively few animals of which we know the entire curriculum 

 vitce. 



70. ON EMBRYOGENY : I. THE GROSSER VISIBLE 



EVENTS 



Premising that we restrict the descriptions to such small-yolked 

 eggs as those of the sea urchin, or the chordate, Amphioxus, the 

 easily visible events of the embryogeny are : (i) The processes of 

 segmentation, in the course of which the relatively large ovum 

 divides by mitosis into a number of small cells. We may 

 arbitrarily limit this process to the phase at which about 1,000 

 blastomeres, or formative cells, have been formed. (2) The 

 process of formation of the germ-layers now begins. (3) From 

 the germ-layers the organ-anlagen are formed and (4) there is 

 then differentiation of the organ-rudiments so that definite tissues 

 come into existence. For simplicity in exposition we describe 

 embryogenesis in this way, but it must be noted, by way of 

 qualification, that the phases (i) to (4) are rather arbitrary ones ; 

 that the cells formed in segmentation and later phases are not 

 really separate units ; that typical germ-layers are not always 

 easily discriminated and that tissue-cells may be differentiated 

 and again de- differentiated. But from the plain, schematic or 

 typical descriptions we may all the more easily approach the 

 essential problems of development. 



jca. Segmentation. For convenience we regard the ovum 



