No. 3.] STUDIES ON CEPHALOPODS. 279 



idea that the ovum of a metazoan represents phylogenetically 

 a protozoan which is supposed to have possessed a radial sym- 

 metry. We must, however, bear in mind that the ovum at 

 the moment of its first separation as a single cell in the then 

 embryonic body of the parent organism, is different from the 

 ovum at the stage ready for cleavage. If by the growth of an 

 organism is meant an increase in the volume of the original 

 protoplasm and the development of the external configuration, 

 and the cleavage of its material is regarded as a secondary feat- 

 ure which may be dispensed with in the strict definition of the 

 term as applied in the organic kingdom, then the history of the 

 ovarian ovum is just as much a growth in all of its essential 

 features as the conversion of the segmented ovum into an 

 embryo, and the embryo into an adult organism. 



The ovarian ovum increases in size ; it acquires protective 

 apparatus around it ; it develops a particular external configura- 

 tion ; and, above all, it accumulates a greater or less amount 

 of yolk-substance for future consumption. The ovum at the 

 time it is ready for cleavage is not the same organism as when 

 it was first formed inside of the maternal tissues, any more than 

 the two segment stage of the same ovum is the same organism 

 as it is at an advanced stage of its ontogeny. There is a 

 growth and development in the unicellular phases of the one, 

 as there are growth and development in the polycellular phases 

 of the other. When, therefore, one says that the ovum of a 

 metazoan is the phylogenetic representation of its protozoan 

 ancestry, we may ask whether one refers to the primitive ovum 

 first found in the embryo, or to one advanced to the stage ready 

 to divide, between which, we must remember, there exist various 

 stages of growth and development. 



From the developmental standpoint the origin of the ovum is 

 just as old as many of the functional tissues of the parent 

 organism. It is true that the growth of the ovarian ovum is 

 an extremely slow process compared with other tissue-cells, but, 

 nevertheless, it is a growth, and a continuous one, and directly 

 depends on the nourishment and protection afforded by the 

 tissues of the parent organism. Even if we admit that the uni- 

 cellular ovum, irrespective of its stages of growth, represents 

 actually the condition of the ancestral protozoan, a highly dif- 

 ferentiated axial symmetry of a certain metazoan ovum cannot 



