10(3 Proceedings of the Royal Physical Society. 



radially. In later stages, however, the nuclei assume an irregular shape 

 and are scattered over the surface of the yolk. 



While these " autoplasts " (Lankester) are yet in the act of forming 

 the yolk epithelium, a thickening of the periphery of the blastoderm takes 

 place. Transverse sections at this stage show a single layer of cells 

 (Fig. 28) ; at the periphery of this layer cells are dividing, the spindle axes 

 being directed obliquely. After this peripheral proliferation of cells forming 

 a ring on the lower surface of the blastoderm, a layer of cells gradually 

 grows from it towards the centre of the blastoderm, like the closing of a 

 diaphragm. The spindle axes are parallel to the surface of the blastoderm, 

 therefore the formation of the inner layer can hardly be termed a delamina- 

 tion (Metchnikoff, Ussov), or an inbending of the outer layer (Bobretzky). 

 In-growth is a more suitable descriptive term. 



It is not quite precise, however, to describe the inner layer as having the 

 shape of a ring. Bruce already observed, but erroneously interpreted this 

 stage (" mesoderm-bands "), of which Vialleton gave the following descrip- 

 tion : — " La zone obscure 1 ne presente pas partout la merne teinte, mais elle 

 parait formee de deux moities plus sombres seqmrees l'une de l'autre sur la 

 ligne mediane par le cercle central et par des espaces clairs situes au- 

 dessus et au-dessous de ce dernier." I can endorse this, as well as 

 Teichmann's account, though the latter only observed one " espace clair " 

 the indication of the stomodceum. The other part of the blastoderm where 

 the lower layer of cells hesitates to grow for a time, but which disappear 

 again, I should like to interpret as a reminiscence of the proctodoeum. 



In our preparations we have, finally, a stage when the epithelium (or 

 syncytium) arising from the " autoplasts " has inserted itself under the 

 blastoderm, thus entirely enveloping the yolk, and the outer layer of the 

 blastoderm has grown peripherally to form a second sheath round the yolk. 

 The inner layer of the blastoderm, situated between the outer layer and the 

 yolk epithelium, is restricted to the area of the germ calotte. 



Sepia is a striking example of bilaterality persisting from the ovum 

 (Lankester, Vialleton, Watase) through the entire development of the embryo 

 to the adult. 



The desideratum in Cephalopod development, a correct interpretation of 

 the origin and fate of the germ layers, is not very easy to supply. In 

 previous memoirs on the subject we encounter a number of conflicting 

 opinions and an unsatisfactory terminology. 



Vialleton was the first to furnish conclusive evidence as to the formation 

 of the yolk epithelium (membrane perivitelline) from the peripheral segments 



1 . 1 rea optica, Ussov. 



