No. 3.] STUDIES ON CEPHALOPODS. 25 1 



the accurate determination of the ovum as a single cell ; (2) and 

 by the identification of cleavage zvith cell-division. 



In regard to the first question, although Schwann, after con- 

 siderable hesitation, inclined to the conclusion that the ovum is 

 morphologically a cell, and that the germinal vesicle is its 

 nucleus, still he felt compelled by the evidences available in his 

 time to admit that " it was impossible to decide the question 

 whether the germinal vesicle be cell or nucleus." 



In regard to the second question, he was totally unaware of 

 its true significance, as his theory of the Cytoblastema very well 

 shows. Within a few years of the publication of Schwann's 

 great work, however, it was definitely determined that the 

 cleavage of the ovum is a process of cell-multiplication ; and 

 that in the whole domain of developmental phenomena of animal 

 tissues no cell-formation ever takes place independently of those 

 cells already existing in the organism. 



Following the identification of the germinal vesicle of the 

 ovum as the true homologue of a typical cell-nucleus, and 

 the cleavage of the ovum as a process of cell-formation, came the 

 problem of the promorphological character of the ovum. This 

 is an entirely distinct problem from that of the germ-layers, and 

 is essentially the outcome of a more careful investigation into 

 the early phenomena of egg-development. The existence of the 

 germ-layer theory did not depend on morphological views of the 

 ovum. The germ-layers were recognized and studied before 

 naturalists knew anything about cleavage and the morphology 

 of the ovum. The cytological method deals with facts before 

 the cells arrange themselves into definite layers. It deals with 

 a certain cell or a certain group of cells in an early stage of 

 cleavage, from which different embryonic organs and germ- 

 layers themselves are derived. For example, it traces the 

 mesoderm to a pair of cells, or the future ectoderm and endo- 

 derm to the two-cell stage of development. It seeks to explain 

 the differentiation of the axes of the adult or larval organism by 

 tracing them to certain recognizable axes in the extremely early 

 stages of development, or even in the unsegmented ovum itself. 



It seeks to find out the significance of the different planes of 

 cleavage in their relation to the origin of tissues in the larva or 

 the adult. Thus, instead of starting with the formation of the 

 germ-layers in the study of organogeny, it goes a step further 



