CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



241 



and of the formative yolk ; cleavage is at first limited to the germ- 

 disc. The conditions of this process therefore differ essentially from 

 those in the other Mollusca. The principal features of these pro- 

 cesses have been known for many years, being described in Kolli- 

 ker's famous work on the development of the Cephalopoda (No. 24). 

 These investigations which, from the nature of the case are far from 

 being exhaustive, appeared in 1S4.">, and were followed by observa- 

 tions made by Bobretzky (No. 4), Ussow (Nos. 44-46), Vialleton 

 (No. 48) and more recently by Watase (No. 50). We shall here 

 follow the last-named author, dwelling especially on the detailed 

 descriptions of the processes of cleavage in Lolicjo Pealii as given by 

 him and of Sepia officinalis as described by Vialleton. So far as 



r/t.^. 



B 



v 



K. 



V 



/. 



Fig. 107. — Two germ-discs ol Sepia officinalis showing the first (J) and second (//) 

 cleavage-planes and the polar bodies (rk) (after Vialleton). 



is at present known, cleavage seems to take place in the different 

 Cephalopods in very much the same way. 



The spindle of the first cleavage-nucleus which is preparing for 

 division lies with its longitudinal axis running from right to left 

 (Watase), and therefore in the plane depicted in Fig. 106 A* 

 It therefore, curiously enough, does not appear to lie, according to 

 the usually accepted rule, in the direction of the widest extension of 

 the formative protoplasm. 



The first line of cleavage runs, in correspondence with the position 

 of the spindle, from before backward, cutting the axis of the spindle 

 at right angles. The first furrow thus lies in the median sagittal 



* Cf. p. 239 on the bilateral symmetry of the egg before cleavage. 



R 



