TEXT-BOOK OF EMBRYOLOGY. 



a nucleus. This form of cell division, which was considered by Remak and his 

 associates (1855-1858) as the only method by which cells proliferated, is now 

 known to be of rare occurrence. Flemming goes so far as to state that m the . 

 higher animals amitosis never occurs as a normal physiological process in ac- 

 tively dividing cells, but is rather to be considered as a degeneration phenomenon 

 occurring in cells whose reproductive powers are on the wane. It frequently 

 results in nuclear division only, the cytoplasm remaining undivided, thus giving 

 rise to multinuclear cells. It is a common method of cell division in the 



Protozoa. 



(2) Mitosis.— In this form of cell division the cell passes through a series 

 of complicated changes. These changes occur as a continuous process, but 



for clearness of description it is convenient 

 to arbitrarily subdivide the process into a 

 number of phases. These are known as the 

 prophase, the metaphase, the anaphase, and 

 the telophase. Of these the prophase in- 

 cludes the changes preparatory to division 

 of the nucleus; the metaphase, the actual 

 separation of the nuclear elements; the 

 anaphase, their arrangement to form the two 

 daughter nuclei; the telophase, the division 

 of the cytoplasm to form two daughter cells 

 and the reconstruction of the two daughter 

 nuclei. 



Prophase (Fig. 3). — In actively divid- 

 1 h, 2.— Epithelial cells from ovary of ing cells the centrosome, or, more specific- 

 ally, the centriole, may be double (Fig. 3, 

 A), having undergone division as early, fre- 

 quently, as -the anaphase of the preceding division (p. 6). Each centriole 

 is surrounded by a clear area, the centrosphere, from which radiate the 

 delicate astral rays, the whole being known as the attraction sphere (Fig. 3, 

 B, C, D). Connecting the two centrosomes are other delicate fibrils forming a 

 structure known as the central or achromatic spindle (Fig. 3, B, better developed 

 in C and D). The two centrioles with their surrounding centrospheres, astral 

 rays and connecting spindle, constitute the amphiaster. If the resting cell 

 contains only one centriole, division of the centriole with formation of the 

 amphiaster is usually the first phenomenon of mitosis, the connecting central 

 spindle fibers appearing as the centrioles move apart. 



During or following the formation of the amphiaster, important changes 

 occur in the nucleus. It increases somewhat in size and the reticulum charac- 

 teristic of the resting nucleus becomes converted into a single long thread 



cockroach, showing nuclei dividing ami- 

 totically. Wheeler. 



