430 M. N. Cholodkowsky 07i. the 



morpliological value of its appendages and their homologies 

 with extremities of other Arthropods. Herein the homology 

 of the anterior end of the embryo in all Arthropods is 

 assumed ; the homology of the posterior end is out of the 

 question, for the number of the abdominal segments varies 

 greatly in different Arthropods. 



In setting up homologies of the parts of the body and the 

 extremities the question of the value of the foremost cephalic 

 appendages is of special importance, for it is precisely on the 

 basis of the conception of these appendages that attempts 

 have been made to divide the type of the Arthropods into 

 two, three, or four subtypes. In the critical examination of 

 the morphological value of the appendages the innervation of 

 the latter is also taken into account, and justly so. I have 

 no intention of enumerating here the attempts which have 

 been made to homologize the cephalic appendages of Arthro- 

 pods, since this would lead me too far ; it will be sufficient 

 to allude to the fundamental principles of these homologies, 

 which have been accepted by the majority of authors as 

 dogmas. Thus it is considered to be an established fact that 

 (1) the head of Insects consists of four metameres ; (2) the 

 antennae of the Tracheata, partly by reason of their inner- 

 vation from the supra-oesophageal ganglion, are to be regarded 

 as pre-oral appendages ; (3) the chelicerge of the Ai*achnida 

 (which were formerly held to be homologues of the Insectan 

 antennge) are homologous with the mandibles of Insects, since 

 they are originally innervated from a post-oral ganglion, 

 which only subsequently fuses with the supra-oesophageal 

 ganglion ; (4) the first (anterior) pair of Crustacean antennae 

 is homologous with the antenna3 of Insects, since to the 

 second pair of antennae there corresponds a special pair of 

 ganglia which is originally post-oral, though it subsequently 

 fuses with the supra-oesophageal ganglion. 



Certain highly important facts have recently become known 

 which, in my opinion, render the justice of the above view of 

 the cephalic appendages of Insects very doubtful. In 

 Chapter IV. of this memoir (p. 43) I have alluded to the 

 fact that the conjecture has already been expressed by Ticho- 

 mirow * that the Insectan head perhaps consists of six meta- 

 meres ; further, that in the case of Chalicodoma even as many 

 as seven embryonic cephalic segments are supposed to exist 

 by Carri^re, and that I myself on the basis of my own inves- 

 tigations am inclined to consider that not less than six 

 segments are present in the head of Insect embryos. The 



* A. Tichomirow, ' Entwicklungsgesch. des Seidenspiuners im Ei ' 

 (Moskau, 1882j : in Russian. 



