THE MOSAIC STAGE OF DIFFERENTIATION 



219 



an oil cap, a clear zone, a yolk field, and a zone of pigment; this 

 stratification may bear any relation to the original axis of polarity. 

 Nevertheless, normal larvae develop, regardless of the distribution 

 of the visible contents. ^ The same is true of the egg of Chaetopterus, 

 and the polar lobe may contain any of the visible materials without 

 influencing normal development.^ 



A B 



Fig. 105 



Section through a frog tadpole (external gill stage) developed from an egg centri- 

 fuged for 5 minutes at about 1500 revolutions a minute. A, Through the head 

 region. The brain (b.) is represented by a degenerate mass of pigment cells. The 

 cranium (cr.) is rudimentary; hy. hyoid; br.c. branchial cleft. B, Through the 

 trunk and the spinal cord (sp.c); the distribution of cells is abnormal, and the 

 spinal ganglia (sp.g.) are fused below it; int., intestine. In both, the myotomes 

 (my.) are fused in the middle line. (After Jenkinson, Quart. Journ. Micr. Sci. 

 LX, IQ15.) 



By way of contrast with specific materials of the organ-forming 

 type as seen in Dentaliiim and Styela, the preformed substances 

 such as yolk and fat to be found in many eggs thus appear to play 

 the part of raw materials only. Their importance as regards normal 

 development is perhaps best illustrated by the experiments of 

 centrifuging the eggs of the frog. When a frog's tgg is thus treated, 



int. 



Morgan, 1910. 



Lillie, 1906. 



