1 70 WHITMAN. [Vol. I. 



The larval epidermis arises in precisely the same manner as the 

 epidermis of Clepsine ; it is lost during larval life, and replaced 

 by the "definitive'' epidermis, which is an entirely separate and 

 independent formation from the neuro-nephric stratum, having 

 absolutely no direct genetic relations with the original epi- 

 dermis. The origin of the definitive epidermis, as described 

 by Bergh, has no parallel in other animals, and it is clearly im- 

 possible to reconcile it with my observations on the fate of the 

 neuro-nephric stratum. The mode of reconciliation suggested 

 by Bergh (No. 20, p. 6), according to which the epidermis of 

 Clepsine is the homologue, not of the larval, but of the defini- 

 tive epidermis of the Gnathobdellidae, must be set aside as en- 

 tirely incompatible with the facts presented in this paper. 



There can be no doubt about the accuracy of Bergh's obser- 

 vations on the loss of the larval epidermis ; but- his theory of 

 the origin of the definitive epidermis I am not able to accept 

 on the evidence adduced. I have shown that three of the four 

 rows of cells constituting the neuro-nephric stratum of each 

 band are employed in the formation of the nerve-chain and the 

 nephridia. I have not been able to satisfy myself fully as to 

 the fate of the fourth or lateral row; but I have followed it far 

 enough to ascertain that it has nothing whatever to do with the 

 formation of the epidermis. The epidermis is perfectly distinct 

 at every stage from the neuro-nephric stratum, and I cannot 

 discover the shadow of a reason for thinking that it ever re- 

 ceives any contributions from this stratum. 



In this connection I must mention one fact which links the 

 teloblasts with the epidermis. I have shown that the neuroblast 

 is the twin cell of the median nephroblast. The mother-cell 

 (No. I , Pi. XII., Fig. 3 5 , x^^ , before dividing into these two cells, 

 produces a small median cell (-^'^), which, together with its 

 homotype of the opposite side, is converted directly into true 

 epidermal cells. This pair of epidermal cells (x'^) is a constant 

 and striking feature of the stage referred "to. This is the nearest 

 point of connection between the epidermic layer and the neuro- 

 nephric stratum. But I venture to say that no one acquainted 

 with the development of Clepsine would risk the suggestion that 

 the whole epidermis is derived from this median pair of cells. 



Kleinenberg (No. 26, p. 129) confirms Bergh's statement as 

 to the loss of the original epidermis in Nephelis, but raises objec- 



