248 WATASE. [Vol. IV. 



is the method of such analysis, and the achromatic spindle the 

 instrument used in the analysis. The cleavage of the squid 

 ovum will then be described, and finally variations in the cleav- 

 age of the same animal will be discussed. 



The observations were made during the summers of 1888 and 

 1889, while I was a student at the Johns Hopkins University, 

 at the Woods Holl Laboratory of the U. S. Fish Commisson, 

 through the kindness of Professor M. MacDonald. 



The pressure of other studies at the time and the difficulty of 

 obtaining a constant supply of the most desirable stages have 

 left my observations incomplete in many respects. Although the 

 squid eggs are abundant during the summer months at Woods 

 Holl, it is difficult to get the early stages, since the bunches 

 of egg-capsules caught in a dredge or in a trawl are usually too 

 far advanced for the study of cleavage. The following observa- 

 tions were made on njaterial obtained from two female squids. 

 On the night of September 3, 1888, one squid, Loligo Pealei, 

 kept in an aquarium, laid four capsules of eggs. The eggs were 

 examined immediately, and the cleavage stages were followed. 

 All the figures on Pis. IX and XH, except Fig. 29, were drawn 

 from specimens obtained from this source. Subsequently I 

 killed the same squid and fertilized the remaining ova artifi- 

 cially. A few better drawings of the surface views of the divid- 

 ing eggs were thus obtained. 



One day in August, 1889, another specimen of the female 

 squid with ripe eggs was kindly given to me by Professor F, 

 M. McFarland of the Marine Biological Laboratory. The eggs 

 were taken out and artificially fertilized with spermatozoa which 

 were found in the spermatophores in the inside of the external 

 buccal membrane of the same animal. All the figures in Pis. X 

 and XI, and Fig. 29 in PL XJI, were drawn from this material. 

 Judging from my experience, I believe that the study of the 

 early stages of cleavage is best accomplished with artificially 

 fertilized ova. Such ova have many advantages over those laid 

 naturally. In the first place, the eggs can be studied free from 

 the jelly covering, which interferes in no small measure with the 

 process of successful manipulation. In the second place, the 

 ova may be obtained at any stage of their cleavage at the time 

 when it is most convenient to work. 



If one succeed in catching a gravid female, it is an easy mat- 



