436 



On the Yelk-sac of Cephalopoda. 



I was mucli struck by the remarkable structure and rhyth- 

 mic pulsation of the sac on the foot of the embryo slug when 

 I first studied it at Jena in 1871 ; and in the winter of the same 

 year, when carrying on researches on the development of the 

 Cephalopoda at Naples, I made the observation, first of all, 

 that the wall of the yelk-sac of the embryo squid is rhyth- 

 mically contractile, and, secondly, that the structure of that 

 wall and its contractile elements is very closely similar to that 

 of the contractile sac on the foot of the embryo Limax. I 

 subjoin outline drawings of an embryo slug and an embryo 

 squid, to render clear to those not familiar with these objects 

 the position of the parts under discussion. 



Fig. 1. 



Fig. 2. 



Fig. 1. Diagram of an embryo Slug. 



Fig. 2. Diagram of an embryo Ceplialopod. m, position of moutb; Ft, 

 foot ; sh, shell ; am, contractile embryonic outgrowth of the pedal region 

 (yelk-sac in Oephalopod) ; oj), eye ; jtk, primitive kidney of slug ; t, 

 smaller head-tentacle of slug ; Fu, funnel of Cephalopod ; mt, mantle- 

 flap of Cephalopod. 



In a paper published in this magazine in February 1873 

 (" Zoological Observations made at Naples in the winter of 

 1871-72 ") I gave a brief outline of my results as to Cepha- 

 lopod development, and I there said (p. 84) : — " An interest- 

 ing phenomenon is the contractility of the walls of the yelk-sac, 

 whi.ch is observed at a very early period, as soon as the first 

 rudiments of eyes, ears, and mouth have appeared. A rhy th mic 

 wave of contraction passes continually along the wall of the sac, 

 at that part immediately in front of the alimentary tube, and 

 doubtless acts so as to cause a circulation of nutrient material 

 in the direction of the young embryo. The tissue wliich 

 exhibits this contractility is of the same structure (stellate 



