TIIK i:.Ml'.HYOLO(iY OK EUVANESSA AXTIOPA. 103 



iinportaiici' in eiiihrvolt^uy. "If there are any structures," says Balfour, 

 "whose identity throughout the Metazoa is not open to doubt, these struc- 

 tures are the ovum and spermatozoon" ; and though embryologists regard 

 tlie germinal layers as presenting homologies almost erpially certain, it is 

 nevertheless the fact that observation indicates that the ovaries in the differ- 

 ent groups arise from different germinal layers. This fact even led Balfour 

 to doubt the fundamental importance of the germinal layers. It is worth 

 while to review what is known of the origin of the primative ova in the va- 

 rious groups of animals. In the Spongiaria certain of the cells of the 

 general parenchyma are said to develoj) into eggs and so they would be 

 considered mesoblastic ; the Cnidaria exhibit considerable apparent varia- 

 tion, but, as the primitive ova cannot be distinguished from the other cells 

 and as they are known to migrate from one layer to another, it is extremely 

 doubtful if anything can be made out with certainty about them ; in the 

 Annelida the ova arise from cells situated in the lining of the body cavity ; 

 in the Nemathelminthes the generative cells have been observed in the 

 gastrula, where they lie in the hypoblastic lining of the archentron, but 

 later they are free in the body cavity ; in the Polyzoa they are apparently 

 mosoblastic and situated in the space between the stomach and the floor 

 of the vestibule ; a single cell in the mesoderm gives rise to the generative 

 organs in the Nematodes ; with Insects the primitive ovaries are a mass of 

 cells situated at the end of the proctodeum and said by some to arise from 

 it, by others to be derived from the mesoderm, while still others trace them 

 back to certain so-called pole cells which originate before the blastoderm 

 is formed ; in the Crustacea the cells orlvins: rise to the o-enerative 



^ coo 



organs arise during segmentation and are first enclosed in the epiblast and 

 still later migrate inwards ; in Vertebrates the germinal cells are first 

 seen in the so-called germinal epithelium from which they seem to have 

 arisen. 



It is thus seen that a great difference appears in the various groups. 

 The only view that appears to reconcile all these statements is this : The 

 germinal cells do not belong to any layer but are the product of some of 

 the first divisions of the egg cell ; they take part generally in the formation 

 of the blastoderm and them migrate into the mesoderm. Probably in 

 most animals, as in Euvanessa, they are indistinguishable from the other 

 blastoderm cells. In only a few animals have the germinal cells been 

 traced back cell by cell to the egg cell and these cases entirely support the 

 view here advanced. In all cases where they are supposed to come 

 from the mesoderm the later stages, comparatively, are the only ones 

 known. 



As the life history of any animal constitutes a cycle, so does the embry- 

 ology : while all the other tissues of the adult animal are more or less 

 differentiated for their several functions, certain cells of the ovary retain 



