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 coalescence 

 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 phyloge- 

 netically two physiologically differentiated forms of a Protozoon ; 

 their coalescence was equivalent to conjugation : the subsequent seg- 

 mentation of the fertilized ovum was the multiy)lication by division 

 of the organism resulting from the conjugation ; the resulting organ- 

 isms, 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 histories, 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 would 

 be given to the questions which must necessarily be raised in the 

 present chapter; but the results of the extended investigations 

 made during the last few years have shewn that these expectations 

 were premature, and in spite of the numerous recent valuable con- 

 tributions to this branch of Embryology, amongst which special 

 attention may be called to those of Kowalevsky (No. 277), Lankester 

 (Nos. 278 and 279), and Haeckel (No. 266), there are few embryologists 



