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AND MORPHOLOGY OF APHIS. 233 



MoUusk. The germ of the Arthropod becomes antero-posteriorly segmented ; the germ 

 of the MoUusk never does so. Prom these two fundamental differences a multitude of 

 others necessarily follow. 



The Articulate embryo is no less markedly separated fi'om that of a Vertebrate animal, 

 although in the latter, as in the former, it is the neural surface which is first developed ; 

 for I know of nothing in the Articulate embryo to be compared with the primitive groove, 

 the chorda dorsalis, and the dorsal plates of the Vertebrate*. They, like the amnion and 

 the aUantois, are, I believe, structures without a representative in the other two sub- 

 kingdoms. 



There is perhaps, as Zaddach maintains, a certain analogy between the primitive seg- 

 ments of the Ai'ticulate animal and the primitive vertebrae (" Urwu-bel" of llemak) in the 

 Vertebrate, but with the commencing differentiation into tissues the resemblance cntu-ely 

 ceases. The appendages of the Vertebrate embryo are more Molluscan than Articulate in 

 their primitive mode of development. Notwithstanding all these great and real differ- 

 ences, however, there appears to me to be one respect in which a most singular analogy 

 obtains between the Vertebrate and the Articulate type : — it is in the construction of the 

 head. 



Adopting, in some respects, the views of Prof. Goodsu-f , I can recognize at least six 

 more or less complete segments in the completely ossified Vertebrate cranium. It is 

 clear that the Vertebrate mouth opens Uke that of the Articulate animal, though on the 

 opposite side of the body, between an anterior and a posterior set of cephalic segments. 

 In the interior of the cranium a no less natural boundary between the anterior and the 

 posterior set of cephalic segments is afforded by the pituitary body and its fossa, when 

 the latter exists. 



I find, again, in the cranio-facial bend of the base of the cranium in the Vertebrate 

 embryo, something wonderfully similar to the cephalic flexure of the Articulate head, and 

 in the cranial trabeculse (Schadel-balken of Rathke), analogues of the procephalic lobes. 



Wliile fuUy recognizing the fundamental differences between the Articulate and the 

 Vertebrate type, then, I think we should greatly err if we overlooked such singular ana- 

 logies as these. Future research wiR show whether they are or are not the outward 

 signs of a deeper internal harmony than has yet been discerned, between the Articulata 

 and Vertebrata. 



Since the present memoir was read to the Society, some additional facts of importance 

 have come to my knowledge. In the fii"st place, my friend Mr. Lubbock, having under- 

 taken to work out the development of Coccus, was led thereby to search for what I have 

 caUed " ovarian glands " in other insects. His results -niU be p\iblished at length else- 

 where ; but he permits me to say that corresponding organs exist in all Lepldoptera, 

 Eymenoptera, Geodephagous and Hydrodephagous Coleoptei'a, Biptera, and most Neu- 

 roptera, while they are absent in Orthoptera, Fulex, Libellulidw, &c., and are all terminal, 



* I therefore by no means agree with what Zaddach says on this subject, or with regard to the homologue of the 

 amnion in Articulata. 



t As expressed in the Edinburgh New Philosophical Journal, 1857, p. 118 et seq. 



