NUCLEI OP ANIMAL AND VEGETABLE NUCLEI. 141 



Strasburger^s work, and consists mainly of observations con- 

 firmatory of previous knoAvledge. 



The similarity in detail of the process of cell-division in 

 animals and in plants which his researches so fully display 

 were to Strasburger suggestive of a community of descent of 

 animal and vegetable cells. This suggestion cannot be re- 

 garded as being weakened by the existence in plants of other 

 methods of cell- development ; for, as Strasburger repeatedly 

 insists, on the ground of the occurrence of various inter- 

 mediate conditions, as well as from the functional analogy of 

 similar parts, free cell-formation is not an original mode of 

 increase, but has probably been derived from the method of 

 division by the suppression of certain stages. 



As to the nature of the nucleus itself, Strasburger differs 

 entirely from Auerbach, in that he regards that organ as a 

 solid structure closely allied to the skin [Hautschicht). The 

 diversity of forms exhibited by nuclei, and the complexity of 

 their functional processes, seem to the former observer quite 

 to exclude an hypothesis according to which they are merely 

 fluid-filled vacuoles. 



In his views of the forces concerned in cell-formation 

 Strasburger is at one with those^ who consider them as exert- 

 ing an attraction on the surrounding protoplasm, by means 

 of which the latter is collected about the nucleus. Mere 

 physical surface-tension, such as determines the shape of a 

 portion of liberated jDrotoplasm existing in a fluid medium 

 with which it is hardly miscible, is, therefore, not concerned 

 in the formation of cells. 



A third recent publication, concerned chiefly with the 

 phenomena of cleavage, is by Dr. Oscar Hertwig.^ The cells 

 examined Avere the ova of the sea-urchin, Towopneustes 

 lividus, fertilisation being effected by artificially mixing in a 

 watchglass the male and female generative elements. 



The granulous unripe ovum is enclosed in a porous 

 capsule, and contains a germinal vesicle with definite walls, 

 which in its turn includes a germinal spot of grey, compact 

 substance capable of staining deeply in carmine. In addi- 

 tion the germinal vesicle possesses an interior reticular 

 arrangement of protoplasmic threads. 



About the time of fertilisation the capsule becomes con- 

 verted into a doubly contoured membrane which has a clear 

 substance immediately subjacent to it. The germinal 



' Pringsheim, Sachs, Tol. 



^ Beitriige zur Keimtniss der Bildung, Befruchtung und Theilung des 

 thierisclien Eies. Morpholog. Jahrbucli 1. 



