No. 2.] FORMATION OF THE FISH EMBRYO. 435 



contact with the blastomere removed, rounds up and continues 

 to segment. Only those cases were described where one blasto- 

 mere had been completely removed by the operation. 



Records of sixty-eight cases were obtained, and in these the 

 early stages of segmentation were followed and embryos reared. 

 About twenty of this number died, however, before the embryo 

 appeared. 



If we take, as an example, an ^g'g in which the first two 

 blastomeres are equal in size, and remove one of the two cells, 

 we find that, after rounding up, the remaining blastomere goes 

 into a resting stage. The egg operated upon then falls behind 

 the normal q%^ in the rapidity of its divisions. 



When the blastomere divides, it does so into two equal or 

 nearly equal parts (25, B)} and the result is in all respects 

 except size a copy of the normal two-cell stage. The furrow 

 of this first division appears always in the plane where the second 

 furrow of the normal egg would lie. 



The second furrows succeed the first and at right angles to 

 it ; and a four-cell stage results like the normal stage of cor- 

 responding segments, except in point of size (25, C). 



The third furrows come in more or less at right angles to the 

 last, but the regularity from this time on is lost (25, D)\ and I 

 have not found comparisons between the small ^^^ and the 

 whole egg in these later stages particularly profitable. Another 

 more regular eight-cell stage (y2-i6) is shown in Fig. 26, taken 

 from another series where the ^rsl normal cleavage was 

 equal. 



In order to detect possible errors in observations made on 

 the living eggs, a number were hardened as soon as one of the 

 first two blastomeres had been removed. Other eggs operated 

 upon were hardened at the two, four, eight, and many-celled 

 stages. These eggs were then cut into serial sections and 

 examined. They showed in every case that the nucleus of one 

 blastomere had been removed with the protoplasm, and only 

 the nucleus (and protoplasm) of the other cell (or its products) 

 remained. 



1 In Fig. 25, y4, the first cleavage gave two unequal cells. The smaller of these 

 was removed, and from the larger the series 25, B, C, £>, developed. 



