46 LIBBIE H. HYMAN. 



logical fact that the more rapidly an organ is respiring, the more 

 rapidly it gives off carbon-dioxide and consumes oxygen, the 

 more rapidly does it function. A case very similar to the heart 

 is that of the digestive tract, in which Alvarez ('18) has shown 

 that the duodenal end of the intestine has the highest irritability, 

 fastest respiratory rate, most rapid rate of contraction, and 

 greatest susceptibility to drugs, of any part of the intestine and 

 that these factors decrease along the intestine. The matter of 

 the cause and sequence of the heart beat will be discussed more 

 fully in later papers, as the gradient is more easily demonstrable 

 in the chick heart. 



7. General Summary of Observations on the Gradients of 

 Teleost Embryos. Before the appearance of the embryonic axis 

 the central region of the blastoderm is more susceptible in Fundu- 

 lus and Tautogolabms, the peripheral region in the cod. It is 

 highly probable that the central region would also be found to be 

 more susceptible in the cod if a sufficiently early stage were in- 

 vestigated; but unfortunately this was not done. It is evident, 

 however, that the high susceptibility of the central region, if ever 

 present in the cod, is lost at a very much earlier stage than in the 

 other two species. In Tautogolabms (Fundulus being unfavor- 

 able for observations) the region of high susceptibility then 

 gradually shifts from the central to centre-posterior regions and 

 finally to a point on the germ ring where the embryo is to appear. 

 In the cod also the region of high susceptibility becomes limited 

 to the region of the germ ring where the embryonic shield subse- 

 quently develops. From this place in both species the region of 

 high susceptibility grows forwards simultaneously with the ap- 

 pearance of the embryonic axis. In the embryos of all three 

 species of fish there are sooner or later two regions of high sus- 

 ceptibility, the anterior and the posterior end, from both of which 

 disintegration extends towards the middle. The posterior region 

 of high susceptibility arises very early in Fundulus, later in the 

 cod, and very late in Tautogolabms. This double gradient exists 

 in both nervous and mesodermal structures, but nervous struc- 

 tures are as a rule far more susceptible. A very high suscepti- 

 bility of the eyes is a marked feature of Fundulus embryos. 



