48 LIBBIE H. HYMAN. 



shield. Sumner concluded that in Fundulus the expansion of the 

 blastoderm is centrifugal since when needles are inserted in the 

 sides of the blastoderm the germ ring fails to expand at the 

 points adjacent to the needles and bays in the germ ring conse- 

 quently appear at those places. Similar conditions seem to be 

 present in the salmon since according to the experiments of 

 Kopsch ('96) the destruction of spots in the germ ring at each 

 side of the earliest stage of the shield does not affect the forma- 

 tion of the embryo. It is evident that this type of development in 

 which the center of the blastoderm is the region of high activity 

 harks back to such a condition as that seen in the ganoids and 

 amphibia in which the animal pole is the region of greatest ac- 

 tivity, as evidenced by more rapid rate of division and greater 

 susceptibility to toxic agents (Bellamy on the frog). 



In the cod, on the other hand, development proceeds in a dif- 

 ferent manner. The expansion of the blastoderm over the yolk 

 is here evidently largely due to the activity of the germ ring. In 

 this fish as contrasted with the other two species, the germ ring 

 becomes visible at a very early stage and it and the embryonic 

 shield are very sharply marked off from the rest of the blasto- 

 derm. My observations further show that the margin of the 

 blastoderm in the cod is already differentiated as a region of 

 high activity before the germ ring is morphologically distinguish- 

 able. The mode of development of the cod by peripheral ex- 

 pansion may be regarded as a specialization from the more 

 primitive type exhibited by Tautogolabrus and Fundulus and 

 probably represents a short cut in development with omission of 

 the original centrifugal method of growth. 



With the expansion of the blastoderm we next observe in Tau- 

 togolabrus, a shifting of the region of high activity from the 

 central region of the blastoderm to the central-posterior and 

 finally to a definite region of the germ ring where the embryo is 

 to form. It seems to me probable that in these changes in the 

 Tautogolabrus blastoderm we have an illustration of the manner 

 in which the centrifugal method of development is transformed 

 into the germ ring type. The conditions in Tautogolabrus thu* 

 eventually come to be identical with those very early present in 



