240 C. O. WHITMAN. 



limits are not y('t visible. That each part into which the 

 germinal vesicle divides represents the centre of a cell, receives 

 an ocular demonstration by the formation of the blastoderm. 

 As cleavage is only the outward expression of a change originat- 

 ing in the nucleus itself, it is all the same whether it appears a 

 little sooner or a little later. But according to the view taken 

 by Biitscldi the cell may pass during the period of proliferation, 

 from the uninuclear to the multinuciear condition, and from the 

 latter back again to the former condition_, without once losing 

 its character as a single cell. 



What then is a cell ? It is no longer a body of protoplasm 

 with a single nucleus, or with any dejlmtc number of nuclei, but 

 one in which the number of nuclei may vary without specified 

 limits. According to this, any tissue in the state of syncytium 

 (Hackel), whether produced by the formation of free nuclei, or 

 by a simple concresence of originally distinct cells, might be 

 called a cell. 



It is obvious that any definition of the morphological unit we 

 call a cell, capable of general application, must be based on some 

 constaut element of the same. The nucleus is a constant, and 

 in the vast majority of cases at least a single element of the cell. 



Biitschli (^tI4tt)> ^"^ harmony with his theory that the egg 

 and cleavage-spheres pass through the multinuciear to the uni- 

 nuclear condition, is inclined to regard the former as the more 

 primitive, and to see in its recurrence after each cleavage, a 

 repetition of the ancestral form of the cell. This view is 

 supposed to be supported by another fact, viz. that in some 

 multinuciear Infusoria the several nuclei coalesce before the 

 division. In reply to this, it may be said that the multinuciear 

 condition is not the earliest condition of tlie egg-cell. The in- 

 vestigations of Engelniaiin (tttt-t) arid Zeller {^^%, 0|)alina) 

 make it certain that in one Infusorian at least the multinuciear 

 is preceded by the uninuclear condition. 



Opposed to the inter[)retation of these corpuscles as nucleoli 

 is the colossal size attained by them in the egg of Rana (^^-). 

 And yet the absence of anything in them that could be called 

 nucleoli, and the fact that the fine granular substance surround- 

 ing them takes the lead in the process of division, as Gotte has 

 shown (tt-Vt)? are opposed to the view that they are nuclei. 

 Oellacher (^t'i^tttt) has given an interesting account of similar 

 bodies which he found in the early cleavage-stages of the egg of 

 Salmo fario. These " clusters of nuclei " were supposed to 

 arise by repeated division of the first cleavage-nucleus, and each 

 was regarded as a veritable nucleus, destined to become the 

 centre of a future cell. No coalescing of these bodies before, 

 and re-formation after cleavage was observed. The whole 



