82 MARINE BIOLOGICAL LABORATORY. 



ovum is a cytode, the next stage in the process of evolu- 

 tion according to Haeckel's scheme, and that it returns 

 for a few minutes to a lower grade of organization simply 

 to indicate its ancestry is certainly an idea at variance 

 with all morphological principles. Thanks, however, to 

 our more perfect technique, we now know that the 

 original nucleus of the ovum does not disappear but is 

 the direct ancestor of the nuclei of all the cells compos- 

 ing the organism resulting by development, and there 

 is therefore no Monerula stage in the developing ovum. 



The next staoe of evolution was the formation of a 

 nucleus, by which the Monerula was converted into a 

 cell or cytode, the ancestral form being a Cytula equiva- 

 lent to the existing Amceba. Following this came the 

 Moj'ula, represented ontogenetically by the morula stage 

 which occurs in the development of certain forms, and 

 during which the embryo consists of an undifferentiated 

 solid mass of cells. That this stage can be considered 

 primitive, and the early appearance of a segmentation 

 cavity which is found in so many forms a secondary 

 condition seems however very doubtful. The evidence 

 at our disposal points the other way. No living repre- 

 sentative of the morula stage is known, and to fill this 

 gap Haeckel proposes an hypothetical ancestor, the 

 SynaniosbiiLin. 



The fourth stage is the blastiila, a hollow sphere pro- 

 duced by the usually contiguous cells of the morula 

 secreting a fluid, which, passing to the interior, forces 

 the cells to the periphery. The hypothetical ancestor 

 corresponding to this ontogenetic stage is termed the 

 Plaiicea, 



The fifth stage is the gastnda, formed from the 



