10 KARYOKINESIS. 



(c) . Nucleolus — The chromosomes, which are at first widely scattered through 

 the nucleus, figs. 3 and 4, gather together more closely and often lie immediately 

 around and upon the nucleolus, figs. 5 and 6. In some cases it looks as if these 

 chromosomes were being formed out of the substance of the nucleolus, and the fact 

 that the nucleolus diminishes in size as the chromosomes increase lends color to this 

 view. On the other hand, when the chromosomes first appear they are scattered 

 through the entire nucleus and do not lie close to the nucleolus, and though it is 

 possible that they may later receive substance from the dissolving nucleolus, it is 

 impossible to suppose that they are fragments of the nucleolus. The latter is grad- 

 ually dissolved without any fragmentation. Before the complete disappearance of 

 the nuclear membrane the nucleolus has greatly diminished in size, and at the same 

 time the nuclear sap stains more deeply, fig. 5. After the disappearance of the 

 nuclear membrane the nucleolus comes to lie outside the spindle, while most of the 

 chromosomes are found within it, though some of them may still be scattered among 

 the polar radiations, fig. 7. 



Within the cytoplasm the nucleolus continues to diminish rapidly in size and 

 soon entirely disappears, figs. 8, 9, 12a. In this respect the history of the nucleolus 

 in the first maturation of Crepidula is the same as has been described by Hacker 

 ('93), Foot ('94), Mead ('95), Wheeler ('95), Obst ('99), and others, in a considerable 

 number of animals. 



(d). Chromosomes. — The shapes of the chromosomes of the first maturation 

 spindle are shown in figs. 7-15 and in text fig. I. In the early prophase the most 

 common form is that of a 3- or 4-lobed body ; in fact, such bodies are found in 

 the nucleus of the ovarian egg. There can be little doubt that these are the 

 "tetrads" of authors, though in Crepidiila they are not alwa\-s 4-lobed. As these 

 chromosomes increase in size a hole appears through the middle, between the lobes. 

 There are also found circular or elliptical rings which may be completely closed or 

 may be open on one side ; also dumb-bell and cross-shaped bodies. All these forms 

 are represented in text fig. I ; all the 2-part chromosomes are shown in the fii'st line 

 (A), the 3-part ones in the second line (B), and the 4-pai't one in the third (C). 

 These forms are grouped according to evident resemblances merely and it is not cer- 

 tain that they always stand in the genetic relations indicated. For example. A, 4 

 and 5 may give rise to B, 6 and 7 ; B, 4 and 5 may be only variations of C, 3 and 

 4, etc. In all cases, however, the short chromosomes of the early prophase give rise 

 to rod-shaped or elongated chromosomes in the metaphase. In some cases (e. g. line 

 A) this is probably accomplished by these chromosomes becoming ring-shaped and 

 by the opening of these rings on one side. If the ring shows no thickenings (A, 2 

 and 3) a rod-shaped chromosome is formed by its opening, which later becomes 

 dumb-bell shaped. (A, 4 and 5); if it shows three thickenings (B, 1-4) it gives rise 

 when opened to a rod with a thickening at each end and one in the middle (B, 5). 

 The 4-part chromosomes (C, 1) are frequently drawn out into cross-shaped ones; 

 these crosses usually have a hole through the middle and each arm of the cross is 

 split lengthwise from this hole nearly to the tips (C, 2 and 3). In later stages the 



