PRINCIPLES OF ANIMAL BIOLOdY 21 



B. MITOSIS IN THE SEGMENTING EGG OF ASCARIS 



Mitosis may be readily studied in the segmenting egg of A scan's 

 megaiocephala (a roundworm parasitic in the intestine of the horse), 

 in the skin of young salamander larvae, or less satisfactorily in the seg- 

 menting egg of the white-fish. The description below applies directly 

 to Ascaris, but may be modified to apply to the others. 



A knowledge of the nature of the specimens of Ascaris in which 

 mitosis is studied will obviate some confusion. The salient features 

 follow : 



(a) The segmenting eggs are in the uterus, a tubular organ, which 

 is cut in thin sections. If the sections are cut longitudinally, each rib- 

 bon shows the walls of the uterus at the edges, with the eggs between. 



(&) The eggs have been fertilized, so that in the earliest stages two 

 nuclei (egg nucleus and sperm nucleus) are present. 



(c) The eggs are turned in all possible positions, so that only one, 

 or both of the nuclei may show, also the later division figures may be 

 observed in various positions. 



(d) Each section includes only fractions of the eggs, so that only 

 portions of the nuclei or spindles may be present, or these structures may 

 be wholly lacking. 



(e) After the first division of the egg, only certain of the cells divide 

 in the same manner as the first segmentation. The directions below 

 apply to the first division and later ones of the same kind. 



Directions for Study 



1. Resting Nucleus. Study a cell not undergoing division. Note 

 the nuclear membrane; the net-like arrangement of the chromatin in the 

 nucleus; and one or more net-knots (thickenings in the chromatin network) . 

 How is the chromatin distributed through the nucleus? What is the 

 appearance of the cytoplasm? 



Draw a cell with resting nucleus, showing also the nature of the cyto- 

 plasm. The thick membrane around the egg, and at some distance from 

 the egg may be omitted. 

 2. Prophases. 



2a. Find a cell in which the chromatin is arranged in distinct but 

 still slender threads (fine spireme) . Where are these found in the nucleus? 

 Observe the nature of the cytoplasm. Look for a darker mass, the 

 attraction-sphere, in the cytoplasm. A dark central granule may or may 

 not be visible in this mass. 



26. Select a cell in which the chromatin is in thick worm-like 

 strands (coarse spireme). Is the nuclear membrane still present? If 

 so, where in the nucleus are these strands? Observe the cytoplasm. 

 The attraction-sphere may be divided into two parts near together, each 



