732 PHYSIOLOGY. 



utricle and the nucleus has been already spoken of in treating 

 of those bodies (see pp. 25 — 27). 



■ b. Free Cell-formation tvithout a previous nucleus. — In the pro- 

 cess of free cell-formation, as described above, we have alluded 

 to the production of the nucleus as the first step of the process, 

 and it is regarded to be so in most instances by the greater 

 number of observers. Henfrey, however, does not consider the 

 nucleus of any physiological import in free cell-formation, which 

 process he thus describes : — " The new cell is formed by a portion 

 of the parent primordial utricle separating itself from the rest of 

 the protoplasm, assuming a globular or oval form, and secreting 

 a cellulose membrane upon its surface, so as to form a new cell, 

 lying free in the cavity of the parent primordial utricle." In 

 some cases, it is certain, no nucleus can be detected in a cell 

 previous to the formation of other cells free in its cavity ; hence 

 the presence of the nucleus cannot be regarded as essential, but 

 the portion of protoplasm, which in such cases separates from the 

 general mass, must be capable of covering itself with a membrane 

 and forming a cell. This, according to Mohl, frequently occurs 

 in the formation of the spores of the Algse, &c. 



In Flowering Plants free cell-formation only occurs in tho. 

 embryo-sac, in which part, after impregnation, both the germinal 

 vesicles and the cells of the albumen (endosperm) originate 

 according to this method. In Flowerless Plants it is regarded 

 by some observers as the mode by which the spores of Lichens, 

 and some of the Algpe and Fungi, originate; by Henfrey and 

 others, however, their formation is believed to be due to a modi- 

 fication of the process of cell- division. 



In the ordinary course of vegetation, free cell-formation can 

 only take place in the protoplasmic fluid contained in the in- 

 terior of cells forming parts of living tissues, but, accord- 

 ing to Schleiden, Mohl, and others, "it may also occur in- 

 dependently of the life of the parent plant in the creation of 

 parasitic Fungi, Yeast cells, &c., both in the decomposing fluid 

 of cells, and in the excreted or expressed juices." 



B. Cell-division. — This mode of cell-formation is also called 

 by authors jparietal and meristnatic or fissiparous cell-formation. 

 It may be treated of under two heads ; namely : 1. Cell-division 

 tdtkout absorption of the walls of the parent cell ; and, 2. Cell- 

 division with absorption of the walls of the parent cell, and the 

 setting free of the new cells. 



a. Cell-division without absorption of the walls of ths parent 

 C(V. — This mode of cell-formation was first observed by Mohl, 

 whose opinions were afterwards ably supported by Henfrey 

 und Mitscherlich. According to these physiologists (and their 

 ob.-^ervations have now been confirmed in all essential parti- 

 culars by various subsequent observers), this process is the one 

 by which all vegetating or growing parts of plants, whether 



