DEVELOPMENT OF THE SQUID. H 



size, and a^thougli the drawing is less enlarged, the embryo shown in fig. 12 is actually- 

 much larger than that shown in fig. 10. The external yolk sac shares in this growth, and 

 is very much larger at a somewhat later stage than the whole egg was at the beginning of 

 the process of development. 



The points of difference between Loligo, Sepia, and Grenacher's embryo are more 

 conspicuous at this stage than at any previous time. 



KoUiker's figures 20, 21, 23, of taf 11, represent about the same stage as our fig. 12, 

 and a comparison will show a remarkable difference in all the details of structure, while 

 the general plan remains much the same. 



The body of the embryo is very much smaller as compared with the external yolk, 

 and is not separated from this by a constriction, but is flattened down upon it. The 

 blastoderm does not yet cover the yolk, but only extends a short distance beyond the 

 circle of arms. Five pairs of arms have made their appearance in the Sepia embryo, and 

 only three pairs are present in Loligo. The outer and inner siphon folds of each side 

 have been united to each other from their first appearance in Sepia, but in Loligo they 

 do not unite until after the two inner folds have united on the middle line to form the 

 mouth of the siphon. The mantle and mantle-cavity of Sepia are about like those of 

 Loligo. Grenacher's figs. 8 and 9, taf 40, are in about the same stage of development; 

 and comparison will show that while this form is more like Loligo than it is like Sepia, 

 the differences are still numerous and considerable. 



The external yolk is very small, and hardly projects beyond the tips of the arms. 

 Grenacher, p. 454, is inclined to regard it as entirely wanting, but his " stirntheil," 

 in figs. 7 and 8, is clearly the same as the external yolk of Loligo, and this again 

 is obviously the same as that of Sepia. His argument that since the internal yolk of 

 Sepia is divided into three regions, three divisions must be found in the internal yolk of 

 his form, and that the " stirntheil " being necessary to make up the three, must therefore 

 belong to the internal yolk, cannot carry much weight ; for at this period the internal 

 yolk of Loligo is only divided into two regions, although a third afterwards becomes 

 recognizable. As regards the external yolk, then. Sepia, Loligo, and Grenacher's embryo 

 form a series, with Loligo as the intermediate form. Grenacher's embryo differs from 

 Sepia, and agrees with Loligo, in having only three pairs of arms at this time. The eye- 

 stalks are about equally prominent in the three forms, and the period of greatest promi- 

 nence is about the same in all ; but while the prominence began to be conspicuous in 

 Loligo and Sepia at about the same time, its development is retarded in Grenacher's embryo 

 until some time after the siphon folds have all united. The outer and inner siphon folds 

 iinite with each other in Grenacher's embryo sometime before the two inner folds meet on 

 the middle line ; whUe in Loligo, the two inner folds not only meet but unite with each 

 other before they join the outer folds. In this respect Grenacher's embryo is intermediate 

 between Loligo and Sepia, since the two sets of folds are united with each other 

 from the first in the latter form. As it has four separate folds at one time, Grenacher's 

 embryo would seem to resemble Loligo more than it does Sepia, so far as the formation of 

 the siphon is concerned. 



At the stages shown in figs. 8 and 9, the mantle of Grenacher's embryo covers 

 almost half the body, and its surface is thickly set with chromatophores, while the mantle- 



