ANIMAL EVOLUTION 21 



these are removed corresponding deficiencies will occur 

 in the larvae reared from the eggs operated upon. 



Weismann himself has modified his former opinion, 

 and in his most recent book on the Evolution Theory- 

 suggests that particles emitted by the nucleus co-operate 

 with elements already present in the cell-body to give 

 rise to specific tissues. That this must be in some sense 

 true, we are bound to believe from a consideration of 

 many phenomena of cell-division and inheritance, but 

 I will not pursue this subject any further now. I have 

 only introduced it to show that experiment has estab- 

 lished beyond all doubt that specific organ-forming 

 substances are present in the cytoplasm of the cell, and 

 the experiment I am going to describe in detail shows 

 that, in a number of cases at least, these substances are 

 dealt out in the course of the division of the germ-cell, 

 just as aces, kings, queens, knaves, &c., held in the hand 

 of the dealer, are dealt out to the different players in 

 a game of cards. 



Fig. I in the diagram shows a picture of the egg-cell of 

 Dentalium, the elephant-tooth-shell. This animal is a 

 mollusc, the representative of the class Scaphopoda, and 

 possessed of many remarkable anatomical features. For 

 the present all that need concern us is, that in the ontogeny 

 of this animal there is a larval form (as indeed is the 

 case in most moUusca), whose shape is shown in Fig. 10, 

 It is shaped something like a humming-top ; having 

 three parallel equatorial rings of ciliated cells, dividing 

 an upper from a lower hemisphere. At the summit of 

 the upper hemisphere is an apical organ, consisting of 

 a group of a few cells bearing a tuft of very long cilia. 

 The greater part of the head region and the ' brain ' 

 of the adult are formed from this apical organ. The 

 lower hemisphere is at first conical, with a posterior 

 tuft of cilia. Nearly the whole of the body of the 

 adult is formed out of the lower hemisphere and the 

 ciliated rings are provisional organs which become 



