244 Thomas H. Montgomery, Jr., 



vicinity of tlie first polar body (Fig. 15, second polar spindle to the 

 right, to the left chromosomes of the first polar body). It is small, 

 apparently without centrosomes, and is found in stages from 

 25 minutes to one and a half hours. Figs. 15 (at the right) — 19 

 show lateral views in successive stages. Fig. 20 exhibits on oblique 

 pole view the two plates of chromosomes of the anaphase, each with 

 12 elements. The second polar spindle never moves to the surface 

 of the egg, but remains below the first polar body within the 

 columnar layer of yolk, or even at the level where this layer meets 

 the deeper layer. The latest anaphases of the second polar spindle 

 are shown in Figs. 21 and 22, in each of which the first polar body 

 {Pol 1) is at the surface of the egg, the second polar body (Pol 2) 

 within the columnar layer of yolk, the egg nucleus {Egg. N) on a 

 line which marks the boundary of the two yolk layers, the small 

 mass of cytoplasm around each of the nuclei is marked by a sur- 

 rounding line. The last stage seen of the second polar body, with 

 the possible exception of some questionable mitoses to be mentioned 

 later, is shown in Fig. 27c, PI. 5. from an egg of the age of 2 hours 

 and 49 minutes; it is a distinct nucleus with chromatin reticulum 

 placed near the surface of the egg, and similar in volume to the 

 egg nucleus (Fig. 27a) of the same egg. 



Thus the first polar body does not seem to be cut off" from the 

 egg, while the second polar body forms considerably below the 

 surface. 



During this time the sperm nucleus moves nearer the centre of 

 the egg; Fig. 23, PI. 4, reproduces it at 5 minutes and Fig. 24 at 

 one hour, after oviposition, which shows that in the first hour it 

 undergoes no marked structural change. The next stage found was 

 that of Fig. 25, at two hours, where it has swollen into the sperm 

 pronucleus. 



V. The Pronuclei and Cleavage Nuclei. 



At about two and a half hours the sperm nucleus and egg 

 nucleus lie close to each other at the centre of the egg (Figs. 26, 

 28 — 30). There is always a size difference between the two, and 

 the larger one is certainly the sperm nucleus because it is always 

 similar in volume to supernumerary sperm nuclei when such are 

 present. Except for this difference they are optically alike, each 

 with a chromatin reticulum and a few small plasmosomes. The two 

 are placed within a common cytoplasmic mass (Fig. 29), within 



