354 INVERTEBKATA CHAP. 



situated. The Cephalopod egg is, in fact, the beau ideal of a telo- 

 lecithal egg. The egg of Sepia is nearly spherical, about the size of 

 a pea ; it is enclosed in a tough black chorion of the consistence of 

 india-rubber, difficult to remove. The egg of Loligo, on the contrary, 

 is about the size of an apple pip, and is of an elongated oval shape. 

 Many eggs are laid together immersed in a somewhat tough jelly, 

 which can be partly dissolved by exposure to the action of Eau de 

 Javelle for fifteen minutes. 



For the study of segmentation stages surface views are essential, 

 and for this purpose the most superficial layer, including all the 

 cytoplasm, is removed from the animal pole of the egg by means of 

 a sharp knife, and the skin thus obtained is spread out flat. Even 

 when sections are desired it is inadvisable to endeavour to cut 

 through the whole mass of yolk ; only a small part of the upper half 

 of the egg should be removed and cut into sections. 



The segmentation of Sepia has been carefully described by Vialletou 

 (1888). Miuchin began a renewed study of the subject and made a 

 series of exquisite preparations from eggs preserved in Hermann's 

 fluid, which, however, he did not describe ; but these have been 

 described by Koeppern (1909), and his results confirm in every detail 

 those of Vialleton, whose account we follow here. No such ex- 

 haustive account of the early stages of development of Lnligo is 

 available, but, from what is known of it, it agrees with that of 

 Sepia in every particular. 



According to Vialleton, then, the cleavage of the egg of Sepia is 

 meroblastic, that is to say, it is only the protoplasmic end of the egg 

 which is divided by the cleavage furrows, the yolk being quite un- 

 affected. When the nucleus has divided into four, and two cleavage 

 furrows at right angles have been formed, we have obviously a stage 

 which corresponds to the stage of division of other Molluscan eggs 

 into four macromeres. 



The next cleavage furrow is a circumferential one and cuts off four 

 inner cells, termed blastomeres by Vialleton, from four outer large 

 cells whose lower ends fade into the yolk, which he terms blasto- 

 cones. This stage corresponds roughly to the stage of the formation 

 of the first quartette of micromeres in other Molluscan eggs, but no 

 one has attempted to work out the cell-lineage in a Cephalopod egg, 

 and it will become obvious that many more cleavages are necessary 

 to separate one from another the specific materials of the germinal 

 layers than is the case with the vastly smaller eggs of other Mollusca. 

 For example, in the 32-cell stage of Sepia (Fig. 285), if the blasto- 

 cones be regarded as corresponding to the macromeres, it is obvious 

 that there must be at least 20 macromeres and only 12 micromeres, 

 whereas in a normal Molluscan egg there would be 4 macromeres 

 and 28 micromeres in the 32-cell stage. 



By further radial cleavage furrows the number of blastocones is 

 greatly increased, and by new circumferential furrows the inner 

 portions of these are continually cut off as new blastomeres, whilst 



