c CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



neighborhood of sperm asters. A similar pair of "asters" are shown in fig. 41. 

 On the other hand, the asters shown in fig. 42 are arranged around the male 

 pronucleus, but how they are related to one another can not safely be inferred. In 

 the following section (fig. 43), drawn from the same specimen, a similar radiation 

 occurs around a supplemental sperm head, n. In the four preceding cases it is 

 interesting to observe how perfectly the rays fulfil the alveolar conditions for aster 

 formation as explained by Biitschli. Note in this connection the large size of the 

 alveoli in the immediate neighborhood of the aster. 



(d) The behavior of the germ nuclei in fertilization is similar to that in shark. 

 The sperm which enters the germ in the region nearest to the egg nucleus is the one 

 which accomplishes fertilization; it undergoes the customary form changes while 

 traveling through the germ. In the stage shown in fig. 44 its chromatic material 

 is becoming resolved, and the aster which appears beside it radiates from a 

 centrosome, which is in this case somewhat elongated, situated close to the 

 nuclear membrane. A stage somewhat earlier than the foregoing is shown in fig. 

 45 ; this, however, represents a stage in the development of a supplemental sperm 

 head. The foregoing figures are taken largely from late stages in fertilization. A 

 stage from a nearly finished capsule (fig. 46) pictures the union of the germ nuclei, 

 /. e. , corresponding to Riickert's fifth stage in the fertilization of the ray (Torpedo), 

 as figured in the Kupffer Festschrift (fig. 53 B). On the other hand, fig. 46 A, 

 which at first sight suggests copulating pronuclei, must be construed as picturing a 

 (sperm) merocyte dividing amitotically ; for here a third nucleus is found to be pres- 

 ent, above the niveau of the other two. The figure indicates, further, the retention 

 of the aster and an extensive pale-colored area surrounding the nuclei. 



(e) The behavior of the supplemental sperm heads is also notably shark-like. In 

 even the middle stage of fertilization they can not readily be distinguished from the 

 early sperm nucleus. Indeed, the nearer they are in a position to the egg nucleus 

 the more difficult they become to distinguish from one another. And conversely 

 those undergo the least conspicuous changes which occur in the margins of the 

 germ. We have already referred, in fig. 45, to a structure which from its position 

 is apparently the early sperm nucleus. In this phase, at the margin of the nucleus 

 is a vesicular area, at one end of which an aster radiates from a minute centrosome. 

 A somewhat similar appearance occurs in what, from its eccentric position, is 

 undoubtedly a supplemental sperm head (fig. 45 A). Here the vesicular area of 

 the nucleus is less perfectly developed, strands of karyoplasm passing from the 

 nuclear membrane to the large and deeply staining mass of chromatin, a stage, 

 indeed, which may be looked upon as the earlier condition of that of fig. 45. 

 Another sperm head (fig. 45 B) from the same series of sections is intermediate 

 between those of figs. 45 and 45 A. The vacuolated margin is now broken into 

 several discrete areas, and the chromatin is collected into a diffuse mass, irregular 

 in outline.* From this stage the transition is not wide to that of fig. 34, in 



*The aster lies below the plane of the section. 



