172 H. V. WILSON AND BLACKWELL MARKHAM 



to such an embryo toward normality. Another way would be 

 complete division. 



3. The lateral halves of the blastopore lip organize into half- 

 bodies, and these come together sjrmmetrically in the midline to 

 form a single embryo. 



4. The lateral halves of the blastopore lip organize into half- 

 bodies, but one atrophies while the other gives rise to an entire 

 trunk. Or one lip fails to organize, while the other does so, first 

 into a half-, then into a whole trunk. The two variants inter- 

 grade. 



This paper describes two cases which fall in the fourth group. 



II 



EMBRYO OF BUFO LENTIGINOSUS WITH ASYMMETRICAL 

 REGULATION 



Eggs of Bufo had been artificially inseminated February 27th, 

 and from a culture the great bulk of which were developing nor- 

 mally, the embryo in question was picked out March 4th. In 

 this embryo (fig. 1), which has begun to lengthen, the neural 

 plate is conspicuous, but the yolk plug is still large. The medul- 

 lary folds are both continued backward on the left side of the 

 blastopore, ending posteriorly in a little projection, p.n., which is 

 distinctly lifted above the neighboring surface. The axis of the 

 neural plate is thus not straight, but posteriorly inclines some- 

 what to the left, while the blastopore has been pushed over toward 

 the right. In the middle of the posterior (ventral) lip of the 

 blastopore, and therefore at some distance from the posterior 

 end of the neural plate, there are two very small knobs, t.h., 

 representing the tail buds. 



The embryo was sectioned, and figure 2 represents a section 

 taken transversely through the posterior end of the neural plate 

 and therefore across the yolk plug. The archenteron, arch, is 

 large. The notochord has formed. The neural groove is pres- 

 ent, with a medullary fold, Imf and rmj, on each side of it, the 

 left one the larger. The gastral or notochordal mesoderm, nc.m., 

 is distinguishable on each side of the notochord, on the right pass- 



