A. W. BELLAMY. 



(g) By the shifting of the blastopore so that it may come to 

 lie at or above the equator of the egg. (Accompanies d, e, /.) 



(//.) By the apparent obliteration of bilaterality, resulting in 

 the formation of embryos radially symmetrical about the original 

 polar axis. 



(i) In the later embryonic stages a variety of abnormal types 

 appear most of which are later stages of c, d, e and g, above. 

 Among them may be mentioned, microcephalic forms with eyes, 

 nasal pits, and ventral suckers in all degrees of approximation 

 to complete "fusion"; anencephalic forms appear, and spina 

 bifida of all degrees are common. Most of the forms show a 

 marked dorsal concavity and the medullary folds may remain 

 open in the brain region or throughout their entire length. 



5. Since the dorsal lip region arises at the distal end of a rapidly 

 growing region, and bears a definite spatial relation to the 

 apical region that is experimentally controllable, it is probable 

 that it arises by physiological isolation consequent upon the 

 rapid growth in length, especially in the sagittal plane. 



6. The data presented indicate that at the beginning of de- 

 velopment, metabolic processes are most rapid at the apical 

 (animal) pole and decrease toward the basal (vegetative) pole. 



7. With advance in development a secondary region (dorsal 

 lip) of high susceptibility appears, probably as a physiological 

 isolation and represents a posterior "segmental" region of rapid 

 growth similar in many respects to the posterior growing region 

 of annelid larvae. 



8. It is pointed out that the existence of gradients in metabolic 

 rate along physiological axes in the egg and embryo make possible 

 the great uniformity and lack of specificity shown to exist in the 

 susceptibility relations in the frog and other organisms, and 

 affords the basis for a rational interpretation of teratogeny, 

 not only in the frog but also in vertebrates generally. 



VII. LITERATURE. 

 Bataillon, E. 



'01 Etudes experimentales sur 1'Evolution des Amphibiens. Arch. Entw.- 



Mech., 12: 610-655. 

 Behre, E. H. 



'18 An experimental study of acclimation to temperature in Planaria doro.o 



cephala. BIOL. BULL., 36: 277-317. 



