December i, iSgS-l 



SCIENCE. 



301 



Occasionally a nucleus is found in which all that is to 

 be seen within the nuclear membrane is this reticulum, 

 without local aggregations of the chromatin. In the 

 greater number of nuclei the chromatin is so disposed 

 that certain local thickenings may be observed. Under 

 a power of about 500 diameters these accumulations of 

 chromatin appear to have no connection with the nuclear 

 membrane, but each nucleus seems to have a well- 

 defined nucleolus. Under a power of 1,200 diameters, 

 however, the connection between the strands of the 

 reticulum and this central body stand out clearly. This 

 aggregation of chromatin may be condensed, and in 

 some instances may be described as spheroidal ; in other 

 more numerous instances it is elongated, and, with its 

 radiating strands of the reticulum, looks very much 

 like a bone lacuna, with rather coarse canaliculi. Usually 

 but one such body is found in a nucleus ; but occasion- 

 ally there are two side by side, or both near the niiclear 

 membrane, and it is not rare to find four or five. From 

 the behavior of these local aggregations and the strands 

 of the reticulum to haematoxylin, it is not possible to 

 determine a difference. Both have about the same tint, 

 and any slight difference of shade may be attributed to 

 the quantity of colorable matter present in the aggrega- 

 tions. 



In situations where it is to be supposed that cell multi- 

 plication is proceeding rapidly, as in the Wolffian bodies 

 and the inner lining of the cerebral vesicles and central 

 canal of the developing cord, many nuclei are found 

 whose nuclear membranes are indistinct, in many cases 

 invisible. Those nuclei, however, are quite conspicuous, 

 owing to the fact that the chromatin is no longer dis- 

 posed in thin shadowy strands, but is in heavy solid 

 skeins, taking a much deeper stain than any part of the 

 resting nuclei. Moreover, these deeply staining bodies 

 of chromatin in these nuclei assume the position of the 

 nuclear membrane that has disappeared, thus forming a 

 basket with irregular meshes. Thus far I have not been 

 able to determine whether in these nuclei it is a single 

 skein, or a number of segments, that enter into the 

 formation of this basket; but in certain nuclei, where 

 the basket was not very regular, detached segments 

 were certainly determined. In some nuclei in which 

 mitosis was well established the loops of chromatin, 

 or chromosomes, were seen scattered through the 

 nucleus, as if the basket had been broken into frag- 

 ments and crushed in. No traces of the nuclear or 

 achromatic spindle were observed before the monaster 

 stage. 



The monaster stage was seen in many nuclei, but the 

 best view was always obtained when the achromatic 

 spindle was lying at right angles to the line of vision. 

 When the aster was seen from the pole the chromosomes 

 were in such a tangle that no satisfactory view was ob- 

 tained. In the nuclei of embryo kittens the chromo- 

 somes are short and thick, and in the haematoxylin em- 

 ployed took a very deep stain, in many cases almost 

 black. For these reasons it was usually impossible to 

 distinguish individual chromosomes in either the mon- 

 aster or dyaster stage, but the ends of the chromo- 

 somes were usually distinct. 



The achromatic spindle at this stage is fairly conspicu- 

 ous and well defined. The chromosomes are seen 

 clustered in the plane of the equatorial plate, while on 

 both sides the fibrils of the achromatic spindle converge 

 toward the pole corpiiscle. From the region surround- 

 ing the pole corpuscles, radiating out into the cyto- 

 plasm, are to be seen the exceedingly delicate rays of 

 achromatic substance, forming the polar cones. Many 

 nuceli were seen at this stage presenting the appearance 

 of the conventionalized diagrams, such as Quain's 



"Anatomy," tenth edition, vol. I., part II., figure 214, 

 except that the chromosomes were not so distinct as in 

 the diagram. 



In the process of metakinesis all phases were seen, 

 from that in which the limbs of many chromosomes re- 

 mained in contact, while the apices of the loops had 

 separated, to the complete dyaster stage. In some 

 instances the ends of the limbs of two or four chromo- 

 somes remain in contact, the others having separated. 

 In nuclei in which the two sets of chromosomes have 

 migrated for some distance, and are separated by an 

 interval equal to the average diameter of a resting 

 nucleus, the exquisitely fine webs that stretch from the 

 ends of the limbs of one set to the ends of the limbs of 

 the other set may be seen in many instances. When the 

 two sets are separated by a small interval the web is 

 not easily seen. 



In the dyaster stage the two sets of chromosomes do 

 not present the appearance that is usually represented. 

 As stated before, the chromosomes of the cat are short 

 and thick, and the limbs do not extend in such a way 

 as to make it easy to determine their number. It is 

 stated that the nuclei of each species contain a definite 

 number of chromosomes. From what can be determined 

 in the nuclei under observation, each set of chromesomes 

 in the dyaster stage contains four chromosomes, al- 

 though it is difficult to determine this point with 

 certainty. 



The portion of the achromatic spindle between the 

 pole corpuscles and the two sets of chromosomes can be 

 made out easily, as the delicate webs are quite conspicu- 

 ous in the dyaster stage, and seems to take a deeper 

 stain in many instances than in the monaster stage. I 

 am not certain that the webs of the spindle react to 

 haematoxylin, but am certain that in some instances 

 this seems to be the case. The radiating webs beyond 

 the pole corpuscles, extending out into the cytoplasm 

 and forming the polar cones, have not been made out in 

 the dyaster stage. 



The chromosomes in the two-daughter nuclei, then, 

 asstime the basket form. The baskets found in the two- 

 daughter nuclei are easily distinguished from the basket 

 in the initiatory stage of karyokinesis by the fact that 

 daughter nuclei occur in pairs, and each basket is much 

 smaller than that found in the mother nucleus. The 

 meshes in daughter nuclei are also much smaller, and 

 the chromatin is in a close tangle. 



Of all the stages of karyokinesis in these nuclei, the 

 dyaster stage is most conspicuous and most easily found. 

 Mitotic figures are most abundant in embryos about five 

 millimetres in length ; in older embryos they are not so 

 easily found. In examining sections from a five-milli- 

 metre embryo, some fields show karyokinetic figures in 

 fully half the nuclei. 



In these embryos karyokinesis was observed in the 

 following situations : 



I. Lining of primitive cerebral vesicles. Here they 

 were most abundant. Nuclei bounding the cavity 

 showed the figures especially well. 



II. Lining of central canal of the spinal cord. Here 

 also very abundant. 



III. Lining of lumina of tubules of Wolffian bodies. 

 Occasional. 



IV. Epithelium lining the pharynx. 



V. Within the branchial arches. 



VI. Epithelium lining the branchial clefts. 



VII. Optic vesicles. 



VIII. CDtic vesicles. 



IX. Epiblast forming epidermis of face. 



X. Walls of heart. 



