20 ATLAS OF THE FERTILIZATION AND KARYOKINESIS OF THE OVUM. 



IV. THE "PAUSE." FORMATION OF THE CLEAVAGE-NUCLEUS. 



Soon after the stages just described, all trace of the distinction between maternal and paternal chromatin is lost 

 to view, and a true cleavage-nucleus is formed, containing a uniform reticulum of chromatin. The astral rays, mean\vhile, 

 become much shorter and the aster otherwise changes its character. In this condition the egg remains, apparently 

 quiescent, for a considerable period (12-20 minutes), which may conveniently be called the "pause." The most striking- 

 phenomenon occurring at this time is the growth of the nucleus, which may increase as much as five or six times in bulk 

 (cf. Phototypes 4, 21, 23, 24, and Text-figs. X.-XIII.). At the same time the aster changes in its staining capacity, the 

 central mass appearing a clear bright red after double-staining- with Congo red and hsematoxylin, while the rays are 

 clear blue. 



Plate VI. Phototype 21. 



Typical '" PauseT Cleavagc-uuclcus and Asicrs {2^ mimeses). 



This specimen shows especially the asters. The two nuclei have completely fused to form a clcavagc-miclcus 

 traversed by a close reticulum (much better shown in the following figure). (Cf. Text-fig. XIII. A.) 



The asters differ strikingly from those of the last stage, the central mass having greatly increased in size, while the 

 rays are very short and their microsomal structure is much more clearly apparent. The central body of the aster (" astro- 

 sphere," of Fol ; " centrosphere," of Strasburger, "centrosome " of Boveri) is a granular mass staining bright red after 

 ha?matoxylin and Congo red. In some cases (e.g., in the annelid Chatoptems, as described by Mead,' who has kindly 

 allowed me to examine some of his preparations), the central mass of the aster undoubtedly contains at this period one or 

 two deeply staining centrioles, which in this case may possibly have the morphological value of centrosomes. In Toxo- 

 fpneustcs, the centre of the aster often contains one or more deeply staining extremely minute granules (smaller than the 

 microsomes, and relatively very much smaller than the centrioles of C/ict'lopterus). These bodies doubtless correspond 

 to the " centrosomes " of some authors (e.g., Heidenhain), but in this case they appear to be quite inconstant in number 

 and size and to be merely an early indication of the reticulated centrosphere, afterwards developed within the aster. 

 They afterwards increase in number and are then indistinguishable from the general reticulum'-' traversing the centrosphere. 



^ Journ. Morph., .X., i, 1895. 



^ The whole question of the origin and meaning of the "centrosome" is at present in a very unsatisfactory condition. (For a critical discussion see an article 

 by the author in the August number of the Journal of Morphology, 1895, entitled " Archoplasm, Centrosome, and Chromatin in the Sea-urchin Egg.") In Boveri's 

 latest paper (Ueber das Verhalten der Centrosomen bei der Befruchtung des Seeigel-eies, in Verh. d. Phys. Med. Ges. Wurzburg. N. F., XXIX., I. 1895), the word is 

 applied to the entire central mass of the aster, exclusive of the rays — i.e., the '• centrosphere " of Strasburger or the •' astrosphere " of Fol — and the word '• centriole " 

 is suggested for a smaller dark body (" centrosome " of Strasburger) that is often found within it. This terminology is accepted in the present work, though the word 

 " centrosphere " is generally used in place of centrosome, as being less open to misconstruction. The precise relation between centrosome and centriole is still in doubt. 

 It seems certain that the centriole is in many cases a definite morphological body lying within the reticulated centrosphere, capable of growth and division, and 

 showing characteristic staining reactions. But neither can there be any question, in the author's opinion, that the bodies described as " centrosomes" have in some 

 cases been nothing more than artefacts due to a local clotting of the reticulum by the reagents. Such "centrosomes" can easily be produced in the centrospheres 

 of Toxopneiistes by the use of chromic or picro-osmic acids, and Eismond has recently shown (An. Anz. X. 7, 8, 1S94) that the bodies called "centrosomes" 

 (centrioles) in the asters of amphibian blastomeres may have a like origin. It still remains possible that centrioles may exist, even in these cases, but are so minute 

 as to elude detection or are destroyed by the fixing agent, or are not stained, and therefore invisible. There are, however, many grounds for accepting the view 

 that the centriole, though a frequent, is not a necessary, element of the centrosphere. According to Vejdovsky (Ent. Untersuch., I. 1888) the centrosome ("daughter- 

 periplast") is not at first present in the sperm-aster, but arises endogenously within it (I.e. pp. 144, 145); and the centrosome is thus reformed as a centriole at 

 every succeeding division, the original centriole enlarging to become bodily converted into the centrosphere (centrosome of Boveri), while a new centriole appears 

 within it. Sala shows in a recent paper (Arch. Mik. Anat. XLIV'., 111. 1895) that a perfectly characteristic centrosome may be caused to appear at the pole of the 

 polar spindle in Ascaris (in which it is normally represented by a group of granules) by an abnormally low temperature. Heidenhain, who believes that the centrosome 

 (centriole) is an absolutely essential element of the aster (Arch. Mik. Anat., XLIII., p. 651) admits that their number varies, and that they may arise de tiovo. — 

 i.e., not by division or budding from a pre-existing centriole (I.e. pp. 654, 655). Watase considers the centrosome as merely a modified form of the cyto-microsome 



