486 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [32] 



with the margins more attenuated than when the formation of the disk 

 was first completed. At the middle of its shortest diameter a transverse 

 furrow now appears, which, by the eighth hour after impregnation, has 

 caused the disk to assume an hour-glass shape when observed from above 

 or below, the two halves being almost exact counterparts of each other. 

 If one will now arrange the mirror of the microscope so as to cause the 

 light to fall upon the eggs obliquely without passing up the tube to the 

 eyepiece, the transparent germinal disk will be found to inclose a very 

 large number of 'tine granules, which show a disposition to arrange them- 

 selves in a definite manner. These granules are found to have aggre- 

 gated somewhat towards the center of each half of the disk, with a clearer 

 space in the central portion, as shown in Fig. 9, Plate II. The clearer 

 spaces n in the opposite halves of the disk may be regarded as the nu- 

 clei of the two segments which must have resulted from the first seg- 

 mentation nucleus inclnded in the germinal disk before it had exhibited 

 any sign of division. The first cleavage may now be regarded as com- 

 plete, and if the reader will observe the granular bands running across 

 the middle of tiie disk in Figs. 9 and 10, he will notice clear spaces be- 

 tween them. This is due to an equatorial arrangement of the granules 

 of the germinal matter and indicates the point of separation between 

 the two cells resnlting from the first cleavage. The first cleavage of 

 the disk may now be considered complete. 



In an hour and a half more, as shown in Fig. 10, the second cleavage 

 has been completed and is indicated by an emargination at either end 

 of the disk and the differentiation of granular bands and a clear space 

 along a furrow traversing the disk at right angles to the segmentation 

 furrow of the first cleavage. The granular bands in both cases being- 

 due to the same causes, namely, a polarity which is manifested in the 

 process of cell division in general, from which it results that the gran- 

 ular matter of the protoplasm is arranged in the form of a kind of 

 double partition or plane coinciding with the direction of the cleavage 

 fun-ow. The cell-plate so defined is represented in Fig. 10, but is not so 

 easy to make out after the segmentation has advanced still farther so 

 as to divide the disk into much smaller segments. We also for the last 

 time have the nuclei distinguished when the disk has been cleft into 

 four segments, afterwards these are not discernible without the use of 

 reagents. 



In the course of two to four hours more still further advances have 

 been made in the segmentation or cleavage of the germinal disk, but 

 usually in a very regular way, the segmentation farrows cutting each 

 other at right angles and dividing each of the four cells of the first and 

 second cleavages into at first two and then four masses of nearly equal 

 size. By the twenty-third hour after impregnation, the germinal disk 

 of the cod's egg will have been divided into fourteen to eighteen seg- 

 ments, as shown in Fig. 12 from below and 11 from the side. In Fig. 

 12 the large cell at the right is just beginning to divide, the incipient 



