TEXT-BOOK OF EMBRYOLOGY. 



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

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

 known to be of rare occurrence. Flemming goes so far as to state that in th 

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

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

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

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

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

 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, bi 



for clearness of description it is convenier! 

 to arbitrarily subdivide the process into t 

 number of phases. These are known as th-- 

 prophase, the metaphase, the anaphase, an ! 

 the telophase. Of these the prophase ii 

 eludes the changes preparatory to divisic 

 of the nucleus; the metaphase, the actu* 

 separation of the nuclear elements; tr : 

 anaphase, their arrangement to form the two 

 daughter nuclei; the telophase, the divisio* 

 of the cytoplasm to form two daughter eel 

 and the reconstruction of the two daught' 

 nuclei. 



PROPHASE (Fig. 3). In actively pr 

 FIG. 2. Epithelial cells from ovary of Hferating cells the centrosome is, as alrea( 



cockroach, showing nuclei dividing ami- 



totically. Wheeler. noted (p. 3), usually double (Fig. 3, & 



having undergone division as early, fr 



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

 some is surrounded by a clear area, the centrosphere, from which radiau 

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

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

 structure known as the central or achromatic spindle (Fig. 3, B, better develope<i 

 in C and D) . The two centrosomes with their surrounding centrospheres, astra ' 

 rays and connecting spindle, constitute the amphiaster. If the resting c< 

 contains only one centrosome, division of the centrosome with formation of tl 

 ani])hiaster is usually the first phenomenon of mitosis, the connecting centr 

 spindle fibers appearing as the centrosomes move apart. 



During or following the formation of the amphiaster, important chang 

 occur in the nucleus. It increases somewhat in size and the reticulum chara 

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



