Eugene Howard Harper 33 



A difficulty is presented in finding the division stages, on account of the 

 surrounding yolk. It must be remembered that these nuclei are not in 

 the favorable region for observation in the center of the disc where the 

 yolk granules are fewer, but they are migrating through the deeply stain- 

 ing surroimding region. Hence during the division stages when at their 

 minimum size they are to be seen only under the most favorable circum- 

 stances. Several clumps of chromatin threads in the spireme and later 

 stages were seen, enough to indicate the probable presence of other divi- 

 sion stages. The resting nuclei are, of course, larger and easily seen. 

 For the above reason the details of the first division of the nuclei have 

 not been made out in the very limited material at hand. This difficulty, 

 however, later disappears, and the mitotic division of these nuclei may be 

 made out with perfect ease. 



If the sperm nuclei divide before the cleavage nucleus, then the rate of 

 division of the latter is a resultant of a slower and a faster rate. The rate 

 of division of the unfertilized egg nucleus may be considered as approach- 

 ing zero. There have been contradictory observations as to whether the 

 unfertilized blastodise of the chick may segment parthenogenetically. 

 Appearances have been observed which at any rate suggest this. Barfurth, 

 94, offers a different explanation of these phenomena holding that there 

 is no true cleavage in the unfertilized eggs. Assuming that possibly it 

 may occasionally happen that the unfertilized egg nucleus may divide for 

 several generations of cells, it is in accord with the accepted view as to 

 the nature of the sperm protoplasm that the sperm nucleus shoidd show 

 a faster rate of division than the egg nucleus, and that the fusion nucleus 

 should have a somewhat slower rate than that of the sperm nuclei. But 

 Kiickert found in the selachian egg that the sperm nuclei divide syn- 

 chronously or nearly so, with the cleavage nucleus. Oppel, 92, found in the 

 reptilia, on the other hand, that the accessory nuclei divided more slowly 

 or not at all in many cases. It is thus seen that special adaptations have 

 arisen in different groups. Environment seems to have more to do with 

 the division of the sperm nuclei than the nature of their own protoplasm. 



In the course of their further migration the nuclei reach the coarser 

 yolk surrounding the inner zone of the germinal disc. Here, either be- 

 cause of a difference in the chemical nature of the materials surrounding 

 them, or because their progress is impeded by the coarser yolk granules, 

 the nuclei remain and their division is followed by a cleavage of the sur- 

 rounding cytoplasm. The first indications of this accessory cleavage on 

 the surface of the egg are seen when the first furrow is established be- 

 tween the cleavage nuclei (Pig. 40) . The continuation of this cleavage 

 and division of the surface into small cell-like areas is indicated in the 



