22 



ATLAS OF THE FERTILIZATION AND KARYO KINESIS OF THE OVUM. 



V. THE PROPHASES. ORIGIN OF THE KARYOKINETIC FIGURE. 



The origin of the karyokinetic figure (p. 5) involves two processes, viz., i) the formation of the amphiaster or 

 achromatic figure; and, 2) the formation and division of the chromosomes which form the chromatic figure. 



A. The Amphiaster. — The asters of the cleavage-amphiasters arise directly from the corresponding halves of the 

 sperm-aster which have persisted through the " pause." The spindle is formed between them, out of the achromatic 

 network (linin) of the nucleus, as shown in Text-fig. XIII., XIV. At the close of the pause (25-35 minutes) the 

 nuclear membrane suddenly fades away at the poles, and in sections the future spindle-fibres may be traced at this 

 point into tlie interior of the nucleus (Fig. XIII. B), and into continuity with the linin network, now very distinctly 

 differentiated from the chromatin. A few minutes later the nuclear membrane entirely fades away, and a distinct 

 spindle is formed traversing the space between the two asters (XIII. C, XIV.), and completing the amphiaster. 



All the evidence goes to show that the spindle-fibres do not properly grow into the nucleus, as they seem to, 







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Fig. XIII. — The segmentation- or cleavage-nucleus (looo diameters). 



A. The " pause " (thirty minutes). The two germ-nuclei have fused imlistinguishably to form a cleavage-nucleus, traversed by a chromatin network. Some of the chromatin in the 



form of rings. The astral rays are much shorter and more granular. In the centre of each aster is a small group of granules. 



B. Initial stage in the formation of the karyokinetic figure. The nuclear membrane has disappeared at the two poles, and the spindle-fibres are forming out of the linin network within 



the nucleus. The chromatin appears partly in the form of irregular rod-shaped bodies, partly in the form of rings. First indication of the reticulum of the centrosphere. 



C. Slightly later stage, the nuclear membrane fading, the spindle nearly formed. The centrosphere assuming a reticular structure. (Phototypes 21-25.) 



but are progressively differentiated out of the linin network from the aster as a starting-point. This fact explains 

 the contradictory accounts regarding the spindle-formation given by different observers, some of whom describe the 

 spindle-fibres as arising entirely inside the nucleus, others as growing into it from the outside and hence of cyto- 

 plasmic origin. 



As the amphiaster forms, the centrosphere becomes distinctly reticulated and stains red, so as to contrast very 

 sharply with the blue spindle-fibres and astral rays. 



B. The Chromosomes. — The chromosomes are always derived from the chromatic reticulum (chromatin) of the 

 nucleus by a morphological rearrangement of its substance, but the modus operandi differs considerably in different 

 cases. According to the earlier accounts the reticulum is first converted into a long convoluted thread, forming 

 what is known as the skein (Knauel) or spirem. The thread then breaks transversely into rod-like segments which 

 are the chromosomes. Later researches demonstrated the fact, however, that the thread is sometimes not continuous 

 but is from the first composed of separate pieces or segments. 



The latter is certainly the case in Toxopneiis/es, and no true spirem stage exists. As the nucleus prepares for 

 division, the chromatic reticulum undergoes a remarkable change. A large part of it loses its staining power and 

 becomes indistinguishable from the linin network, which accordingly undergoes a very great and rapid increase in 



