THE CELL DOCTRINE. 131 



cleus disappears, to be replaced, however, by a new 

 one, under favorable conditions, which appears to 

 take no part in the subsequent cleavage processes.* 



Recent descriptions of the nucleus include in its 

 anatomy a nuclear membrane, which is described by 

 Auerbach as a somewhat thick, highly refractile, 

 doubly contoured membrane, which appears to be 

 less distinctly separated from the cell protoplasm 

 than from the exterior of the nucleus. This mem- 

 brane is regarded by Auerbach as having been formed 

 about the original droplike nucleus by the differ- 

 entiation of the inmost layer of the protoplasm into 

 a species of interior cell membrane. According to 

 Klein, it is composed of an outer thicker portion, 

 which is the limiting membrane proper, and closely 

 connected with it an inner more or less incom- 

 plete layer, which is a peripheral condensation of 

 the intranuclear network, with which it is connected 

 by longer or shorter threads. According to Auer- 

 bach, the nuclei first appear as clear spaces, vacuoles 



* According to Prof. Strieker (" Development of the Simple 

 Tissues," in vol. iii, of " Human and Comparative Histology," 

 New Syd. Soc. Ed.), precise statement to the effect that the germi- 

 nal vesicle is persistent and becomes transformed into the nucleus 

 of the cleavage cells, has only been made by Johann Muller in the 

 case of Entoconcha mirabilis (Monatsberichte der Berliner Akade- 

 mie, September, 1851). But in recording this fact Strieker seems, 

 as many more recent writers have done, to have entirely overlooked 

 the much earlier observation of Martin Barry, who distinctly as- 

 serts (Philosophical Transactions, London, 1840, p. 529), that " the 

 germinal vesicle does not burst, or dissolve away, or become flat- 

 'tened, on or before the fecundation of the ovum, as hitherto sup- 



