Abjiormal Developuieiit of Toad Ova 41 



swim. Thus when the control tadpoles of the same age as the 

 experiment tadpoles are equivalent in general form to the 12 mm. 

 tadpole of the frog described at some length in Marshall's well- 

 known text-book, the development of the control tadpole is, as 

 a rule, more nearly similar to the newly-hatched tadpole of the 

 frog described by Marshall. 



External Form 



The head is usually abnormal in shape, the anterior end appear- 

 ing shrunken. The coelom is in many of the tadpoles abnormally 

 distended. The tail is usually short, more or less deformed and 

 is often bent in a dorsal direction. 



Internal Structure 



The vascular system is little developed in any of the experiment 

 embryos. The heart usually is S-shaped but is rudimentary in 

 form and may have no continuous lumen. In some embryos the 

 wall of the ventricle is thickened by muscle cells but in none are 

 there strong trabeculae in the ventricle. The chief arteries seem 

 in none of the embryos to be completely developed, although in 

 some there are here and there traces of them. The chief veins are 

 likewise in none of the embryos completely developed although 

 in one embryo the cardinal veins are large. In the liver the capil- 

 laries are sometimes well, sometimes but slightly developed. 

 There are relatively a very few blood corpuscles in any of the em- 

 bryos. These lie in some of the scattered vascular anlages. It 

 is uncertain whether the blood had circulated in any of the embryos, 

 but in some of them it is fairly certain that no circulation was 

 established. In all of the embryos the spaces in the tissues indi- 

 cate a considerable amount of lymph either free in the tissues or 

 confined in lymph vessels. 



Of the central nervous system the brain is the part most con- 

 stantly and deeply affected, but the spinal cord in many of the em- 

 bryos is markedly deformed. The abnormalities consist partly 

 offailure of development or tissue differentiation, partly of irregular 

 growth of tissue, pigmentary degeneration of nerve cells and the 



