EMBRYOLOGY OF CRYPTOBRANCHUS 147 



exhibits a tendency toward radial striation and probably repre- 

 sents the aster. 



The study of the paths of migration of the germ-nuclei and 

 the copulation-nucleus is not quite complete, but indicates that 

 the germ-nuclei come together at a higher level than that occu- 

 pied by the copulation-nuclei shown in the figures. 



The first segmentation nucleus in a resting condition has been 

 found in an egg killed eighteen hours after fertilization; the first 

 cleavage spindle has been found in an egg killed seventeen hours 

 ' after fertilization. 



4. Changes in the blastodisc 



In eggs taken from fifteen minutes to ten and one-half hours 

 after fertilization, cytoplasm is accumulating in irregular patches 

 underlying the animal pole (fig. 51). During this period, practi- 

 cally all traces of the debris of the germinal vesicle disappear. 

 In places, the surface of the blastodisc is sometimes very irregular, 

 almost villous; this may be due to injuries resulting from the actual 

 or attempted entrance of spermatozoa. 



In eggs taken from twelve to eighteen hours after fertilization 

 (copulation nucleus to first cleavage spindle) the cytoplasm is 

 gathering in a broken layer close to the surface of the blastodisc. 

 The beginning of this process is shown in figure 52. In Hyno- 

 bius, Kunitomo ('10) has noted a somewhat similar condition. 

 During the latter part of the period considered the layer of cyto- 

 plasm becomes much thicker than is shown in the figure, but 

 retains its segmented character. 



During the first two hours after fertilization there is a marked 

 increase in the thickness and extent of the blastodisc as a whole 

 (see especially fig. 51). Evidently the greater part of this change 

 takes place before the egg has become oriented with the animal 

 pole uppermost, hence it is independent of any possible sorting 

 effect of gravity acting on the materials of the egg. 



No marked changes have occurred in the lower hemisphere since 

 the egg left the ovary. 



The later changes in the blastodisc lead up to first cleavage and 

 will be considered in that connection. 



JOURNAL OF MORPHOLOGY, VOL. 23, NO. 1 



