278 WATASE. [Vol. IV. 



If, in conclusion, I recapitulate what has been said in a few 

 words, the cleavage of the ovum may be characterized as Analy- 

 sis of different protoplasmic substances which form the bases 

 of different tissues, caryokinesis as the Method, and the archo- 

 plasmic spindle as the Instrument. 



IV. 



Since the work of Kolliker was published in 1844, there have 

 appeared a number of valuable observations on the development 

 of Cephalopods, touching more or less on the cleavage of the 

 ovum, such as those of Bobretzky, Lankester, Ussow, Brooks, 

 and Bruce, and quite recently an important contribution on the 

 cleavage in Sepia by Vialleton. 



So far as the observation of facts are concerned, there is very 

 little chance for error in forms like the Cephalopods. Differences 

 of methods, so far as they come within the limits of approved 

 histological practice, can make but a little difference, particu- 

 larly in the study of such a subject as this, where everything 

 comes out with a diagrammatic clearness. In regard to the 

 illustrations of the different stages in Loligo Pealei which ac- 

 company this paper, I feel confident of their general correctness, 

 as far as they go. Most of the preparations from which the 

 illustrations on my plates were drawn are still in existence, and 

 can be verified at any time. I will not, therefore, enter into a 

 critical comparison of the facts presented by different authors. 

 In regard to the interpretation of the phenomena, differences 

 are naturally expected. Whatever may be the value of the pre- 

 ceding remarks concerning the cleavage of the ovum in general, 

 I will attempt to treat the cleavage phenomena in Loligo from 

 that standpoint. In this point my paper differs from that of 

 any previous worker on the Cephalopod embryology. 



Before we enter into the descriptive details of the present 

 subject, a few points of a more general nature occur to me, 

 which may be introduced here. 



The idea of ascribing a comparatively high. Complex structure 

 to the ovum, such as the differentiation of the axes, etc., in the 

 unsegmented ^g^ is attended with scepticism, or accepted with 

 a certain amount of reserve by more cautious embryologists. 

 It appears to me, this hesitation arises chiefly from the prevalent 



