DEVELOPMENT OP THE SQUID. 7 



the process of segmentation was initiated. The growing edge of the blastoderm is 

 marked by a ridge b, which is ciliated. This stage of development corresponds pretty 

 nearly to the one given by Grenacher (Zur Entwickelungsgeschichte der Cephalopoden) 

 in his figure 3 ; and this figure has been copied and referred to as a trochosphere, and 

 the line of cilia around the edge of the blastoderm has been spoken of as a rudimentary 

 velum. Grenacher does not himself suggest any such comparison, which is certainly 

 unwarranted and without basis, as there is no possibility of an homology between 

 the moUuscan velum and a ciliated line, which, if it be homologous with any part of the 

 body of an ordinary mollusc, can be compared only with the gastrula-mouth or orifice of 

 invagination, a portion of the body which has no connection with the velum in any known 

 mollusc. 



At the stage shown in fig. 6, the embryo has become bilaterally sjrmmetrical about a 

 plane which passes through the long axis of the egg, and the blastoderm has become raised 

 into a circular area, the mantle, m, on that end of the egg where segmentation began. 



The primitive condition of the mantle, as shown by the figure, will be seen to be quite 

 different from its condition "at the same stage in Grenacher' s embryo, as shown 

 in his fig. 3, in which the mantle area is indicated by the presence of numerous 

 chromatophores, while the margins of the area are not at all marked, and the elevation 

 from the general surface of the yolk is very slight. In our species, the chromatophores 

 do not make their appearance until much later, and the margin of the mantle-area 

 is well defined at the time of its first appearance. Judging from Bronn's copies of 

 Kblliker's figures, the mantle of Sepia seems to make its appearance about as it does 

 in Loligo, which it resembles much more than it does that of Grenacher' s form. 



The rudimentary arms are indicated in fig. 6, by a slight ridge or projection, a, upon 

 each side of the embryo, a little nearer the nutritive pole of the yolk than the formative 

 pole. The outhne of the yolk, represented by a heavy black line in the figure, is 

 no longer regularly rounded, but begins to conform to the shape of the body of the 

 embryo, a decided prominence projecting into the mantle-area ; and a large, rounded, 

 projecting eminence on each side of the body in the space between the arm a, and the 

 mantle, m, marking the position of the future eye-stalk. 



The history of the later stages of development shows that the end of the egg which 

 is uppermost in this figure, and which is the end where segmentation began, becomes 

 what is usually spoken of as the posterior end of the body of the adult, the extremity 

 opposite the head. The surface which is shown in the figure, is that which is usually 

 called ventral, the surface which carries the siphon. The lower end of the figure 

 is that which is occupied by the head in the adult animal. 



Without entering into a discussion of the homology of the Cephalopod body, which 

 I shall return to at the end of this paper, I shall, for the present, — accepting the 

 views of Leuckart and Huxley, which I beheve to be essentially correct, — speak of 

 that surface which is uppermost in fig. 6, as the dorsal surface ; of the opposite end, 

 as ventral ; of the surface shown in this figure, as posterior ; and of the surface opposite the 

 siphon, as anterior. Fig. 6 is therefore a posterior, instead of a ventral, view. 



Figure 7 is an opposite or anterior view of a somewhat older embryo. The mantle, m, 

 is more elevated, and its margin is quite sharply defined, and nearly circular, when seen 



