METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS, 47 



Regions of high susceptibility also appear in connection with the 

 development of the auditory vesicles and the cerebellum. A 

 gradient exists in the heart (Fundulus}, the venous end being the 

 most susceptible and the susceptibility gradually decreasing to the 

 arterial end. 



III. RELATION OF THE GRADIENTS TO NORMAL TELEOST 



DEVELOPMENT. 



While it is not my purpose to enter into a detailed account 

 of teleost development it may not be amiss to point out the bear- 

 ing of the observations recorded in the preceding section on the 

 normal course of development. In discussing this matter it is 

 necessary to bear in mind the fact that teleost development is 

 highly specialized. Embryologists agree in general that the 

 primitive mode of vertebrate development is illustrated by the 

 ganoids and amphibia, forms with total unequal cleavage. The 

 teleost mode of development is derived from some such type and 

 has probably proceeded along different lines in different groups 

 of teleosts. 



In the early stages of the blastoderm it was found that in 

 Fundulus and Tautogolabrus the central cells have a higher rate 

 of activity, as measured by the susceptibility method, than the 

 marginal cells, while in the cod the margin of the blastoderm is 

 more active. These facts indicate that we are dealing here with 

 two different modes of development. In the two first-named 

 species the germ ring and the embryonic shield are poorly differ- 

 entiated and only faintly visible ; in fact I have not represented 

 them in my figures on this account. It is highly probable that 

 in these species the germ ring plays a minor role in the formation 

 of the embryo and that the embryo is produced chiefly through 

 the activity of the central region of the blastoderm. This con- 

 clusion is supported by the observations and experiments of 

 Morgan ('95) on Tautogolabrus and of Sumner ('03) on Fundu- 

 lus. Morgan concluded that the embryo of Tautogolabrus is 

 formed largely of material that has never been part of the germ 

 ring. He also showed that the development is not affected if cuts 

 are made in the germ ring at each side of the early embryonic 



