UNICELL ULAR NA TURE. 5 3 



dissolution, is there any appearance of a multicellular constitution, or in the 

 former instance, more especially, the formation of a distinct germinal layer 

 or blastoderm, the one essential feature and index of all the multicellular 

 animals or Metazoa. It is true nevertheless, although the circumstance 

 has apparently as yet attracted but little notice, that many infusorial forms 

 belonging both to the Ciliate and Flagellate sections of the group, exhibit 

 in connection with that mode of reproduction characterized by the resolution 

 of the primary cell or zooid into a number of sporular bodies, an aspect and 

 plan of organization in no ways to be distinguished from the moruloid or 

 primary segmental condition of the ovum of the Metazoa. Beyond this 

 stage, however, the analogy, or, if it exists, the homology, entirely ceases ; 

 for whereas, in the Infusorium or Protozoon, each segment or unicellular 

 component of the pseudo-morula becomes metamorphosed into a distinct 

 and independent being, in the Metazoon, this primary independence is 

 almost immediately obliterated through the recasting or reconstruction out 

 of the primary segmental elements, or morula, of the true multicellular 

 embryo with its characteristic inner and outer germinal layers or primordial 

 tissues. In the case of the sponges, as shown later on, and notwithstanding 

 the deceptiveness of external appearances, the production of similar pseudo- 

 morulae are associated both with the growth of the free-swimming swarm- 

 gemmules or so-called ciliated larvae, and also with the development in 

 certain types of the characteristic ciliated chambers, or, as they are more 

 usually designated, the ampullaceous sacs of the adult sponge. 



Taking for granted that all infusorial structures possess a unicellular 

 morphologic value only, the very extensive range of complexity compatible 

 with such simple organization, as exhibited by the representatives of 

 the Infusoria, has now to be considered. Among the majority of the 

 older biologists, and with many even at the present day, the conception 

 of a single histologic cell, or of an independent unicellular organism, differs 

 widely from the one that is here advocated. With the former it was, and is, 

 held, firstly, that such a simple cell or unicellular organism must have a 

 differentiated bounding membrane, the cell-wall or primordial utricle ; and 

 secondly, that the same must contain a central denser and more highly 

 refractive mass, or nucleus, which may or may not be associated with a still 

 more minute segregated mass, the nucleolus. As demonstrated, however, 

 by the later school of biologists, and among whom Professor Haeckel's 

 name occupies a pre-eminent position, neither a distinct cell-wall nor 

 a differentiated central nucleus forms an essential or invariable element 

 of cell-organization, be such cell either an independent being, or unit, 

 or an integral constituent only of a compound tissue. In accordance 

 with the results of more modern investigation, the intrinsic value or 

 potentiality of such a cell-structure resides neither in the cell-wall nor 

 in the nucleus, but in the simple protein matter indifferently denomi- 

 nated sarcode or protoplasm, of which the cell-body is built up. With 

 reference to the more or less highly differentiated organization of cell- 



