The Development of the Squid. Loligo Pealii (Lesueur). 



By W. K. Brooks. 



Although several authors have recorded important observations upon various 

 points in the embryology of the Cephalopoda, our knowledge of the outlines of the 

 process of development, as a whole, is almost entirely derived from the accounts which 

 have been given by KoUiker and Grenacher. Ktilliker's paper contains an elaborate and 

 exhaustive account of the external changes which are undergone by the developing 

 embryo of Sepia officinalis, and Grenacher gives an equally valuable and complete history 

 of the embryo of an unknown species of Decapod Cephalopod. Although these two forms 

 of embryos are substantially alike in all essential particulars, they differ so greatly in 

 all the details of the process of development that similar outline sketches of other 

 Cephalopods are greatly needed, and until they are furnished w^e shall not be in a position 

 to discuss the relation between the young forms of this group and the embryos of other 

 molluscs, or to speculate iipon the manner in which the peculiarly direct and complex 

 form of development exhibited by the Cephalopoda has originated. 



The mode of development of the common Squid is essentially like that of Sepia 

 and Grenacher's Cephalopod, but in minor points it is different from each of them, 

 and in many features it seems to be intermediate between them, and although we have 

 a ntmiber of figures, by various writers, of stages in the development of Loligo, there 

 still remains a need for a continuous account of its history as a basis for comparison 

 with other Cephalopods, and I hope this paper will furnish part of the necessary material 

 for a general discussion of the subject. 



The eggs and embryos which are described were, with two exceptions, obtained at 

 the Chesapeake Zoological Laboratory of the Johns Hopkins University, during the 

 summer of 1879, by dredging in water five or six fathoms deep, in the lower part 

 of Chesapeake Bay. They were collected through the aid of the steamer Lookout 

 of the Maryland Fish Commission, under the command of Major T. B. Ferguson, of the 

 United States Fish Commission, to whom I take pleasure in expressing my thanks for this 

 and for much more valuable assistance which our party has received from him. 



Two of the figures (17 and 18, plate 3) are from embryos which were obtained, 

 during the summer of 1876, at Mr. Agassiz's laboratory at Newport. 



The youngest egg which I observed is shown, magnified eighty diameters, in plate 1, 

 fig. 1. It is surrounded by a well-defined wall or egg shell a, outside which was the 

 gelatinous matter of the egg capsule. The egg shell is transparent, elastic, and is 

 reflected inwards a little at one end, around an opening m, which may probably be 

 regarded as a micropyle. As the embryo grows, the space inside the egg shell 



