Ch. XI] EFFECT OF INJURING A BLASTOMERE 



107 



hot needle. The needle is carefully sharpened, and is re- 

 sharpened after each egg is operated upon. It is best to 

 pierce the blastomere in the black hemisphere near the first 

 cleavage-plane. The needle passes through about a half (or 

 more) of the blastomere. When the needle is withdrawn, a 

 greater or less amount of the contents of the blastomere pro- 

 trudes where the blastomere has been injured. The egg after 

 operation is returned to the water. It is necessary to keep the 

 eggs under careful observation, because sometimes the blasto- 

 mere has been only slightly injured and continues to develop 

 more or less irregularly. Such eggs should be removed. 



A B 



Fig. 31. — A. Hemiembryo lateralis. B. Hemiembryo anterior. (After Roux.) 



Roux found that in twenty per cent, of the eggs the unin- 

 jured blastomere lived and continued to develop. This blas- 

 tomere by continued division developed into a form that may 

 be called a " semimorula verticalis," since it is like the vertical 

 half of a normal "morula." "That is to say, it is a hemi- 

 spherical structure with small deeply pigmented cells above, 

 and with larger non-pigmented cells below." The segmenta- 

 tion-cavity is often absent ; sometimes it is represented by a 

 few loosely aggregated cells, and sometimes by a cavity bor- 

 dered in part by the injured half of the egg (Fig. 35, A). A 

 " semiblastula verticalis " then develops with a well-defined 

 segmentation-cavity. A " semigastrula " stage is next passed 

 through. " Hemiembryones laterales " develop from most of 

 these eggs, as seen in Fig. 34, A. This figure shows that 



