66 



CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



section (fii;. 64 a) to the great number of aniitoses occurring throughout the germinal 

 wall, and on the other hand, the absence of mitoses in this region. Referring again 

 to tig. 64, we interpret the cavity a as the archenteron of the earlier stage which 

 has deepened and reached the germinal wall, still preserving its smooth posterior 

 boundary, but dilated anteriorly and fading into a mass of detached cells. And we 

 identify the cavity sc as the segmentation cavity now enlarged and with irregular 

 offsln)ots. Another section of the present specimen shows, near the side, fig. 64", 

 the marginal extent of the dilated archenteron and the obliteration in this region 

 of the segmentation cavity. Its major interest, however, is in contributing data 

 concerning the relation of the blastoderm to the yolk. Especially at the anterior 

 end we observe that the cellular elements, of great size, have recently become 

 detached fi-om the yolk. 



The details in the study of this specimen deserve especial comment, for they 

 indicate an intimate functional relationship between mitosis and amitosis. * To 

 illustrate these conditions a number of details are given in figs. 64 a-n, all drawn 

 from the foregoing specimen, but from selected sections. We may first refer to the 

 character of the merocytes. In a detail of the anterior end of this si)ecimen, 

 fig. 64 A, over fifty merocytes are j^resent, most occurring in the fine yolk, but some 

 in a superficial rift of coarse yolk which spreads inward toward the blastoderm. We 

 observe: numerous amitoses; the masses of fine yolk whose distinct outlines suggest 

 ])olynuclear blastomeres; at one point (near i ) a nucleus surrounded with fine yolk, 

 altogether suggesting a single blastomere; blastomeres formed out of the yolk (2 and 

 3); a large clear blastomere (4) which ai)pears to have budded out of the germinal 

 wall; and (5) a small clear blastomere, which has undoubtedly been derived from 

 the adjacent yolk. At the opposite end of the blastoderm (fig. 64 b), and within it, 

 is a large cell containing many nuclei, some of which are in amitotic division, and 

 similar ap])earances are observed further along in the same section, fig. 64 i and j. 

 In the first of these, i, a large blastomere has broken up into three smaller cells, in 

 the largest of which the nucleus has subdivided amitotically into at least half a 

 dozen smaller ones; in the second, j, a blastomere has divided and in each resulting 



*The merocytes here considered are regarded as products of the segineatatioa nuclei. The diiiiculty, however, 

 in distinguishing fmally Ijetween the merocytes derived from the segmentation nuclei or from the sperm-heads has 

 already been commented on (p. 57). 



Figs. 64 A-N. Details ot sections of preceding stage. (See page 67.) 

 -A. Detail of germinal wall al exlteme anterior end ol blastoderm. 1-5. ceils which are arising, or have recently arisen from the germinal wall. There can be 



little question from the yolk-filled character of some of these that they have recently arisen from the germinal wall (i. e.. they can not be cells whtch are 



being passed into the germinal wall, as His suggests). Such a cell as that indicated at 3. although destitute of yollc material, is so far from the remaining 



cells of the blastoderm that it could only have been budded off from the germinal wall. 

 Ij- Detail of the posterior rim of blastoderm showing the origin of blastomeres from the yolk wall. Observe that some of the cells are filled with coarse yolk: 



others. 3, have relatively little. The cell. 2, iust separated from the germinal wall, contains a number of (amitotic) nuclei. 

 C Detail of wall of germinal yolk. 1 . 2, 3, Nuclei arising amitotically. passing in the direction of the floor of the subgerminal cavity. 4, Blastomere arising 



from the germinal wall. 3, Blastomere undergoing amitosis. 6, Blastomere arising from the germinal wall, and showing aster. 

 D- Origin of blastomeres from the germinal wall. 4 and 3. Blastomeres recently separated. I. 2. and 3, Nuclei about to be passed into blastomeres. 

 A'. Yolk-filled cell arising from the germinal wall, and exhibiting typical mitosis. Adiacent is a blastomere whose nucleus is dividing amitotically. 

 ^. Blastomeres newly arisen from the germinal wall. 

 (•■ Vesicular nuclei in region near surface of germinal wall. 



.//. Vesicular nucleus, undergoing amitotic division, with adjacent vacuolar spaces. 

 /, ./, A , and /^. Cells of blastoderms in some of which amitosis is taking place. 

 iM and ^V. Cells of blastoderm dividing by atypical multiple mitosis. 



