No. 3.] STUDIES ON CEPHALOPODS. 293 



to a much later stage of embryonic development with special 

 application to the facts of organogeny. Newport^ noticed long 

 ago, in the frog, that "about two hours after the completion of 

 the crucial cleft [the second furrow of cleavage] a new series of 

 changes is set up in the &g^. The clefts no longer include the 

 whole circumference of the o.^^, but are confined to the splitting 

 of the larger into smaller pieces, after a binary plan ; and this 

 process does not begin at once over the whole surface, but 

 appears first in a given spot, and then pursues a definite course ; 

 thus each of the two pieces seen from above on one side (behind) 

 of the crucial cleft becomes subdivided, producing four segments 

 on one side of that line, whilst there are only two on the other. 

 When this subdivision is nearly completed, and not till then, a 

 corresponding change takes place in the two segments on the 

 other side (in front) of the sulcus." 



What are the effective causes which produce these differences 

 in different parts of one and the same blastoderm } 



Unless we study different forms more extensively, discussion 

 in this field is not likely to be of any particular value. The 

 following remarks are therefore only provisional ones, with 

 special reference to the results of my Cephalopod study. 



Growth of any two given parts in an organism may be similar 

 or different, according as the inherent nature of the material 

 which constitutes the given parts are similar or different. Two 

 parts fundamentally alike, on the other hand, may grow at dif- 

 ferent times and at different rates, according to the conditions 

 of environment which affect them. In other words, unequal 

 growth may take place in two parts intrinsically similar, one of 

 the parts, however, being more favorably situated than the 

 other. 



By which of these categories of causes may the unequal growths 

 we have seen in the blastoderms of the squid be accounted for .'' 

 To characterize this whole series of variation simply as patho- 

 logical is not an explanation. On the other hand, it has been 

 suggested that the unequal growth such as is manifested in the 

 alternate cleavage of the squid ovum may be explained as due 

 solely to the influence of external conditions. I am, however, 

 disposed to believe that external conditions have nothing to do 



1 Researches on the Impregnation of the Ovum in the Amphibia ; and on the Early 

 Stages of Development of the Embryo. 3d Series. Philosophical Transactions, 1854, 

 p. 241. 



