216 MR GEORGE BROOK ON THE 



The formation of the second meridianal furrow is merely a repetition of 

 what has been described for the first. The protoplasm at the yolk pole again 

 joins that in the germinal area, and a second furrow is formed at right angles 

 to the first. This second furrow, like the first, does not quite reach the base 

 of the germinal protoplasm. When the first two furrows are completed, the 

 protoplasm in the germinal area is imperfectly divided into four segments, 

 which are not denned at the base. There is again a quiescent stage during 

 which a small thickening of germinal protoplasm is again to be seen at the 

 yolk pole. 



The next furrow is equatorial in direction, and simply completes the 

 contour of the four existing segmentation spheres. Before this can be formed, 

 the protoplasm through which it will pass must have so far completed its 

 process of assimilation as to allow segmentation to proceed. Shortly before 

 the equatorial furrow commences, the protoplasm at the yolk pole again joins 

 that in the germinal area, and the furrow is then formed very slowly, and 

 is at first indistinct. It is situated towards the base of the germinal area, 

 and with its completion there are formed four segmentation spheres, which 

 are now isolated from the yolk. Below the furrow a small portion of the 

 germinal protoplasm remains, part of which forms branching processes into the 

 yolk immediately below the segmented portion, and the remainder becomes 

 distributed around the yolk mass. 



There is thus set up a division into two distinct layers, the archiblast and 

 parablast. The archiblast is cut off from further direct communication with the 

 yolk, and goes on segmenting. The parablast comprises that portion of the 

 germinal protoplasm which is not included in the archiblast, and which 

 remains as a connecting area between the latter and the yolk. For the time 

 being it remains comparatively inactive, but later has a very important part 

 to play. 



The time occupied by all these changes is about 9^ hours, at a temperature 

 of 41° to 44° F. There is a slight variation in the rapidity with which the 

 various eggs develop, which becomes more marked as development proceeds. 

 The following table shows the details of the process : — 



Commencement of 1st furrow, 6| hours after impregnation. 

 Completion „ 6f hours 



Commencement of 2nd furrow, 7| hours 

 Completion „ 8J hours 



Commencement of 3rd furrow, 9 J hours 

 Completion „ 9f hours 



It will be noticed that in each case there is an hour's interval between the 

 completion of one furrow and the commencement of the next. According to 

 Kupffer's observations, the first furrow commences about two hours after 



