6 W. K. BROOKS ON THE ' 



blastoderm. He says that "when the cap of klastoplasts (ectoderm), has spread one-third 

 over the egg, its marginal cells grow by a regular increase in size, and consequent 

 fission, taking place equally all round the margin. But before the superficial extension 

 of the cap of klastoplasts has commenced, there appear in a deeper stratum of yolk 

 pellucid nuclei, at first arranged in a circle around the cap of klastoplasts." This is not 

 the case in our species, where the autoplasts appear inside of, instead of outside of, the 

 cap of segmentation spherules. 



Lankester goes on to say, however, that he believes that in the eggs of Loligo there 

 may be, according to season, an increase of nucleated cleavage segments, or, on the other 

 hand, of these bodies — the autoplasts — they being reciprocally vicarious within small 

 limits. If this is the case in one species at different seasons, it is not improbable that 

 there may be still more difference between our species and the one which he studied ; and 

 to this difference may be due the lack of agreement between his observations and my 

 own. 



The outlines of the surface cells are now very obscure until treated with re-agents, 

 but under the action of carmine and osmic acid, they become very conspicuous as shown 

 in plate 1, figure 5. The growing edge is now formed by a row of large polygonal 

 nucleated cells, h, with well-marked outlines iipon the sides which are turned towards the 

 formative pole, but with faint outlines foding gradually into the unaltered yolk a, upon 

 their distal sides. These cells are evidently growing by assimilation of the yolk sub- 

 stance, and are what remain of the pyramids shown in fig. 4. Inside these there is a 

 row of smaller polygonal cells, which are arranged in pairs along the radii of the larger 

 cells of the first row, as in figure 4, but the regularity of this arrangement is not 

 quite so perfect as it was at the stage shown in that figure. 



Inside this there are concentric rows, d, of much smaller cells, which have a somewhat 

 peculiar arrangement ; they are usually irregular pentagons, but some of them have six 

 or four sides ; their shape is nearly triangular, one end being pointed and the other blunt, 

 and they, are arranged in pairs, with the blunt ends in contact. 



The nuclei, which are near the centres of the cells of the series h and c, are at the 

 blunt ends of the cells of the rows d, sp that the nuclei of each pair of cells are nearly 

 in contact. This peculiar arrangement indicates very clearly that each pair of cells of 

 the series d, has been formed from a cell like that in the series c, by fission along a line 

 parallel to one of the original radii of segmentation. 



The condition of the surface cells of the blastoderm, at this stage of development, is 

 thus seen to have been reached in a very simple manner, by cleavage in four ways, as 

 follows : first the blastoderm is cut up by radial lines of cleavage into a number of 

 radiating pyramids ; second, the central ends of these pyramids are cut off by cleavage 

 at right angles to the first ; third, each of the cells thus formed is divided into two by a 

 cleavage parallel to the first ; and fourth, each of the resulting cells is divided into two 

 by a cleavage parallel to the first and third. 



The next embryo which is figured, plate 1, fig. 6, has advanced very far beyond 

 the stage shown in tig. 3, and the position of the body of the future Squid is 

 now indicated. The blastoderm has grown down arovmd the yolk, which is now almost 

 entirely covered, except at a point y, directly opposite the formative pole, where 



