IIOO BUtl^ETlN OF THE BUREAU OF FISHERIES. 



rim to the edge of the cap and inclosing a clearer space known as the segmen- 

 tation cavity. This condition of things, however, does not seem to exist in the 

 egg of the toadfish. 



Miss Wallace (1898) has shown in a short but highly interesting paper that 

 there is no true invagination in this egg, but that the apparent thickening, the 

 appearance of a germ ring or invaginating blastoderm, visible as the proto- 

 plasmic cap grows down over the yolk, is due in part to an ingrowth of cells 

 from the ectodermic layer, but chiefly to the greatly thickened periblast filled 

 with giant nuclei. 



The writer regrets that he can present no photograph of this stage, as these 

 structures can only be brought out clearly in eggs killed for the purpose, while 

 his photographs are all of living eggs. He has had no opportunity to verify by 

 means of sections the conclusions above stated. Eggs properly "killed" seem 

 to show a very distinct germ ring, and, had he had not been aware of Miss 

 Wallace's work, the writer would, from surface views, unhesitatingly have de- 

 scribed the condition as a case of invagination. 



Formation of the embryonic shield. — The cap of cells described above con- 

 tinues to spread over the upper hemisphere of the egg, growing thinner all the 

 while. Presently at one point on the rim there appears a slight flattening 

 tangential to it, causing the cap to become bilaterally symmetrical in one plane 

 only, this plane being the axis of the future embryo. According to Miss Clapp 

 (1891), the notch or bay described below is formed at the central point of this 

 flattening during this stage. So far, however, as I have examined my material 

 I have found nothing to corroborate her statement. No photograph of this 

 stage is presented for the same reason as given above. 



Differentiation of the embryo. — During all the events described above the 

 blastoderm has been growing larger and extending over the yolk. After passing 

 the equator of the egg, the opening, the morphological blastopore, grows smaller 

 and eventually closes. Growth seems to go on less rapidly at the posterior 

 end of the axial thickening — i. e., that connected with the rim of the blastoderm, 

 the anterior free end extending out into the center of the cap — which results 

 in the formation of a little bay at the hinder end of the forming thickening. 

 Then, as the spreading of the cap over the yolk continues, the blastoderm grows 

 backward away from the embryonic rod, leaving it connected with the edge of 

 the cap by a slender cord of cells. This formation of the embryo is, I believe, 

 peculiar to the toadfish. It was first demonstrated from surface views by 

 Miss Clapp (1891), whose work was verified and extended by her pupil. Miss 

 Wallace (1898), who cut sections of the egg in this stage. 



At Woods Hole the axis of the embryo first becomes visible on the seventh 

 day, according to Miss Clapp (1891). Miss Wallace (1898), however, detected 

 an "axial thickening" on the fourth or fifth day. Artificial fertilization not 



