DEVELOPMENT OF THE SQUID. 21 



says, on p. 269, that the arms are tentacular appendages to the body, and does not appear 

 to regard them as the equivalent of any part of the Gasteropod. 



The innervation of the siphon from the pedal ganglion leads him to regard this as the 

 foot, and he concludes that the valve of the siphon is the true foot or protopodium, and 

 the two lateral folds pteropodia. He conjectures that Grenacher's two inner folds unite 

 and give rise to the valve, but this is opposed to Grenacher's account, as well as to 

 my own observations, and there can be no doubt that the two inner folds form the tube 

 of the siphon and the two lateral folds its lateral chambers. The valve appears quite 

 late, fig. 15, V, as an outgrowth from the inner wall of the siphon tube, and there is 

 nothing in its history which gives any reason to believe that it has ever had any other 

 function than that of a valve to the siphon-tube. Von Jherring's homology rests upon the 

 assumption that similarity in the method of innervation implies similarity of origiu, but the 

 bilateral character of the siphon seems to be an objection to its homology with the foot, 

 even if it were in the right position upon the body, and if it is a new structure, as I 

 believe, the origin of its nerves cannot have any profound morphological significance. 



Beaufort, N. C, July 31, 1880. 



EXPLANATION OF THE PLATES. 



All the figures except plate 1, figs. 2, 4, and 5 and plate 3, figs. 19 and 20 were drawn from living 

 specimens, swimming without restraiat within the uncompressed egg, or, after their escape from the egg, in a 

 sufficient quantity of water to allow perfect freedom of movement. As the use of a camera was thus ren- 

 dered out of the question, the drawings are not all upon the same scale. Most of them were made, however, 

 with an amplification of about eighty diameters. 



Plate I. 



Fig. 1. An egg in which the process of segmentation is somewhat advanced, a. Egg-shell ; b, space 

 between the shell and the yolk, filled with transparent albumen ; c, cap of segmentation spherules ; m, 

 micropyle. 



Fig. 2. More highly magnified view of the blastoderm of the egg shown in figure 1, as seen in optical 

 section ; a, b, and c as before ; d, yolk. 



Fig. 3. A more advanced egg, with the blastoderm covering about a quarter of the surface of the yolk; 

 a, b, c, d and m, as in the preceding figures. 



Fig. 4. More highly magnified view of the growing edge of the blastoderm of the egg shown in figure 1 

 after staining with osmic acid and borate of carmine ; a, unsegmented yolk ; b, segmentation pyramids ; 

 c, first row of cells, which have been formed by a separation from the ends of the pyramids; d, second set 

 of cells, which have been formed by the division of cells like c; d', third set similar to d; d", fourth set, 

 similar to d, and d'. 



Fig. 5. Highly magnified view of the edge of the blastoderm of the egg shown in figure 3, after stain- 

 ing with osmic acid and borate of carmine, a, b, and c, as in figure 4 ; d, cells which have just divided. 



Fig. 6. View of the posterior surface of an embryo, in which the yolk is almost covered by the blasto- 

 derm, a, arms ; b, growing edge of blastoderm ; m, mantle ; y, uncovered portion of yolk. 



Fig. 7. View of the anterior surface of a slightly older embryo ; m, mantle ; s, shell area ; si', lateral 

 siphon folds ; e, eye. 



