METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 49 



the cod. The cod has evidently omitted the early stages and very 

 soon arrives at the condition in which the posterior median region 

 of the germ ring is the center of activity. There has thus been 

 produced the typical teleost method of development by means of 

 the embryonic shield. 



In vertebrates like the frog having a primitive mode of de- 

 velopment it is important to note that a region of high activity 

 also develops in the median posterior point of the ' germ ring," 

 that is to say, the dorsal lip of the blastopore (Bellamy, loc. cit.}. 

 The frog, however, also retains the region of high susceptibility 

 at the animal pole, so that two regions of high activity are con- 

 stantly present the animal pole and the dorsal lip of the blasto- 

 pore. In the teleosts the former region either is not present from 

 the first or first develops and is then lost ; but subsequently a new 

 region of high activity develops at this point. It seems therefore 

 that the germ ring type of development is highly specialized and 

 the attempt to interpret the modes of development of other verte- 

 brates in terms of the germ ring is a mistaken effort ; but rather 

 germ ring types should be interpreted as modifications of the 

 method illustrated by the amphibia. 



After the region of high activity has become limited to a 

 median posterior area of the germ ring, it extends forwards and 

 the embryo appears in its center. That the germ ring is actively 

 concerned in this extension appears improbable since the an- 

 terior end of the shield in the cod and all of the shield except the 

 place where it meets the germ ring in the cunner are the regions 

 of high susceptibility. If the forward growth were produced by 

 the germ ring, by a sort of pushing process, then one would ex- 

 pect the posterior end of the shield to exhibit the highest sus- 

 ceptibility. It seems probable that the shield grows forwards 

 through the activity of its anterior end or possibly in some species 

 with the aid of the surrounding cells of the blastoderm. The con- 

 ditions are evidently variable in different species. According to 

 Kopsch ('96) if the embryonic shield of the salmon embryo is 

 destroyed the germ ring continues to close but no embryo is 

 formed. Here evidently the embryo arises solely from the ma- 

 terial of the shield. In Tautogolabrus and Fundulus, however, the 



