THE DEVELOPMENT OF ISOLATED BLASTOMERES 
OF THE FROG’S EGG 
J. F. MCCLENDON 
From the Zoélogical Laboratory of the University of Missouri and the Histological 
Laboratory of Cornell University Medical College, New York City 
WITH TWO FIGURES 
After the numerous proofs furnished by various investigators, 
there is no question that differentiation begins very early in 
development, but the determination of the exact stage at which 
various differentiations begin, has been hampered by technical 
difficulties. The importance of overcoming these difficulties lies 
in the fact that it is only by an exact study of the early develop- 
ment that we can ever hope to know the mechanics of differen- 
tiation. The later stages are so complex that it is doubtful that 
they could be analysed even though the early stages were under- 
stood. 
The frog’s egg, owing to its large size, has been a favorable 
object of study. O. Hertwig,! after pricking one blastomere, 
found that a complete embryo was formed. 
Roux,? in similar experiments obtained a half embryo from the 
uninjured blastomere, and explains Hertwig’s observation by the 
fact that some of these half embryos became complete larve 
by a process of regulation which he called post-generation. This 
might occur in the following different ways: first, the half embryo 
might fold together on the side corresponding to the median plane 
and be transformed into a whole embryo of half size; second, the 
injured blastomere might recover from the operation and develop 
into the missing half; and third, the injured blastomere might be 
Arch. f. Mikroscopische Anat., 1893, XLII, p. 662. 
?Ges. Abh. zur Entwicklungsmechanik der organismen, Leipzig, 1895. 
