CHAPTER XIII. 



GENERAL CONCLUSIONS. 



I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE 



GERMINAL LAYERS. 



IT has already been shewn in the earlier chapters of the 

 work that during the first phases of development the history of 

 all the Metazoa is the same. They all originate from the coales- 

 cence of two cells, the ovum and spermatozoon. The coalesced 

 product of these cells the fertilized ovum then undergoes a 

 process known as the segmentation, in the course of which it 

 becomes divided in typical cases into a number of uniform cells. 

 An attempt was made from the point of view of evolution to 

 explain these processes. The ovum and spermatozoon were 

 regarded as representing phylogenetically two physiologically 

 differentiated forms of a Protozoon ; their coalescence was equi- 

 valent to conjugation: the subsequent segmentation of the 

 fertilized ovum was the multiplication by division of the organ- 

 ism resulting from the conjugation ; the resulting organisms, 

 remaining, however, united to form a fresh organism in a higher 

 state of aggregation. 



In the systematic section of this work the embryological 

 history of the Metazoa has been treated. The present chapter 

 contains a review of the cardinal features of the various his- 

 tories, together with an attempt to determine how far there are 

 any points common to the whole of these histories ; and the 

 phylogenetic interpretation to be given to such points. 



Some years ago it appeared probable that a definite answer 



