SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 441 



EXTERNAL FORM 



Head. In most, but not in all specimens, the head is decidedly- 

 abnormal in form. As a rule the distortion of the head, is due 

 in the main to a failure of normal formation and expansion of 

 the cerebral vesicles, but it may also be due in large part to the 

 failure of normal differentiation of mouth, pharynx and gill slits. 

 Frequently a sucker is formed when the head is but slightly 

 developed but differentiation of a sucker may be relatively behind 

 that of the head, and not infrequently the sucker is abnormal. 



Tail. In most specimens the tail is but a short deformed bud- 

 like process, sharply deflected in a dorsal direction. It may be 

 exceedingly rudimentary, but on the other hand in a few speci- 

 mens it is relatively well developed. 



Trunk. In a large proportion of the specimens the abdomen 

 protrudes markedly from the body. This protrusion is due to an 

 abnormal dilatation of the body cavity. In most specimens there 

 is a well marked dorsal flexion in the dorsal region of the trunk. 

 This seems to be associated chiefly with defects in the develop- 

 ment of the central nervous system. Irregular outgrowths from 

 the trunk, especially in the ventral portion, are not infrequent. 

 These seem to be chiefly finger like projections from the ectoderm. 



INTERNAL STRUCTURES 



Central nervous system. In those instances in which the effects 

 are most profound in the neurogenic tissue, the neural plate may 

 thicken without folding to form a distinct neural groove or if 

 the neural groove is formed it may persist as an open groove while 

 the larvae develop far beyond the stage in which the groove norm- 

 ally has become converted into a tube. In many specimens the 

 neurogenic tissue then, instead of forming a neural tube with a 

 clean cut lumen, gives rise to a more or less clearly defined rod- 

 like mass of cells in which only here and there are to be seen 

 evidences of a lumen. Rudiments of a lumen are usually to be 

 seen in the head but the lumen even here fails in the extreme 

 forms to expand so as to give rise to well marked cerebral vesicles. 



