Yelk-sac of Cephalopoda. 435 



pods and Lamellibranclis with the pen-sac of Cephalopoda, 

 which I showed to originate, like the shell-gland, as an open 

 invagination. I have been careful to point out reasons for doubt- 

 ing the exact equivalence of the two structures (" On the 

 Development of the Pond-Snail, and on the early Stages of other 

 MoUusca," Quart. Journ. Microsc. Sci. vol. xiv. 1874, p. 371). 



Further, I cannot agree with Mr. Brooks in the view that 

 the niolluscan foot is necessarily an " unpaired " organ. It 

 is truly enough a median organ ; but it has necessarily a right 

 and a left side, which in many cases tend to develop as two 

 divergent lobes ; and such growths as " epipodia " are only 

 an expression of this tendency to bilateral development. 



Mr. Brooks regards the arms of the Cephalopod and the 

 funnel as either epipodial or as new and special organs of 

 Cephalopods, whilst he advocates the view that the yelk-sac 

 of Cephalopods represents the "median unpaired" foot of 

 Mollusca, which has accordingly no representative in the adult 

 Cephalopod. 



Mr. Balfour, in his * Comparative Embryology,' vol. i. 

 p. 225, had anticipated Mr. Brooks's speculation as to the 

 identity of the Cephalopod's yelk-sac with the Gasteropod's 

 foot. He says : — " In Cephalopods the position of the Gas- 

 teropod foot is occupied by the external yolk-sack. In normal 

 forms the blastopore closes at the apex of the yolk-sack, and 

 at the two sides of the yolk-sack the arms grow out. These 

 considerations seem to point to the conclusion that the normal 

 Gasteropod foot is represented in the Cephalopod embryo by 

 the yolk-sack, which has, owing to the immense bulk of food- 

 yolk present in the ovum, become tilled with food-yolk and 

 enormously dilated." 



I am unable to agree with the interpretation put upon the 

 facts by Mr. Balfour and Mr. Brooks. I quite admit that 

 the region in the Cephalopod distended by food-yelk is the 

 axial region of the foot ; that is obvious upon the first obser- 

 vation of the facts. But it is another thing to maintain that 

 the projection or outgrowth as such represents the projection 

 or outgrowth in its entirety known as the foot in Gasteropods. 

 In my opinion it does not do so, but is a special embryonic 

 dilatation of the axial region of the foot, and is no more 

 representative of such an outgrowth as the adult muscular 

 foot than is the very remarkable contractile sac on the foot of 

 Limax. 



Had Mr. Brooks compared his embryo squid with an 

 embryo slug, he would, I think, have come nearer to making 

 out the significance of the latter's yelk-sac than he has when 

 comparing it to an embryo of an aquatic Pulmonate. 



