DEVELOPMENT OF THE SQUID. 17 



In a word, the case is hardly too strongly put, by the statement that the developing 

 Dibranch has so completely lost all ancestral features that no traces of theni remain. 



In another point of view our embryo is more suggestive. Although it furnishes us no 

 basis for phylogenetic speculations, it does furnish a safe ground for the discussion of 

 cephalopod homology. 



Homology, as I take it, is the resemblance which is due to common ancestry, while 

 phylogeny is the study of the steps by which specializations of structure have been 

 acquired. It is quite conceivable that a form of life should exhibit its relationship to a 

 remote ancestor, without any indications of the manner in which the divergence from 

 this ancestor has been brought about. This seems to be the case with the Cephaloj)oda. 

 While the Squid embryo fails to give us any information as to the way in which a typical 

 Mollusc has been modified in order to convert it into a Cephalopod, or as to the trans- 

 formations through which it has passed in reaching its present form, it nevertheless 

 clearly shows the fundamental similarity of type which subsists between it and the other 

 Mollusca. 



The precise relation between the organs and regions of the body of a Cephalopod and 

 those of a more typical Mollusc, have always been as obscure as the close natural affinity 

 of these groups is obvious, and the obscurity has been increased by the disposition 

 to regard the various Mollusca as modifications of a highly specialized architype, uniting 

 in itself the characteristics of all its derivations. This tendency still retains a considerable 

 influence over the minds of morphologists, although it is now perfectly obvious that the 

 architype, or proto-mollusc, must have been an unspecialized rather than a highly 

 specialized form, and that the various existing molluscs must have been derived from this 

 primitive form by gradual specialization. The .only basis for an homology of the 

 Mollusca, in the absence of a phylogenetic record, and of transitional forms, is therefore 

 to be sought in the comparison of early stages in the development of individuals 

 of the different groups, at a time when the specializations of structure, characteristic of 

 the adult forms, have not yet made their appearance. 



In his paper on the development of an unknown Cephalopod, Grenacher has made such 

 a comparison, and has attempted to furnish a sound basis for the discussion of the 

 homology between the Cephalopoda and other Mollusca. 



As the result of this comparison, he reaches conclusions, which, being the product of 

 observation rather than of speculation, are much more valuable and natural than any 

 which have been advocated by previous writers, although I believe them to be only 

 partially correct. 



Although his embryological record is very complete, it unfortunately lacks the stages 

 which are of the greatest importance as a basis for generalization, and I have been so 

 fortunate as to fiU this gap by finding embryos which exhibit general molluscan 

 characteristics unobscured by the presence of the distinctive features of the Cephalopod. 

 For comparison with a Gasteropod embryo I give in fig. 19 of plate 3, a diagramatic side 

 view of an embryo at the same stage as that shown in plate 2, fig. 10, but with the rectum 

 and anus represented as they are found at a somewhat later stage. 



The figure, like fig. 10, has the head and the yolk sac y, below ; the so-called posterior 

 end above ; the surface upon which the siphon is to be developed at the left, and the 

 so-called dorsal surface at the right. 



