EARLY DEVELOPMENT IN THE FROG. 335 



to "run wild" and result in an excessive over-development of 

 skeletal structures. 



(d) Meroblastic Cleavage. In exceptional cases, and under 

 the influence of conditions so severe that development stops 

 completely in early cleavage, cell division may be partly or 

 completely restricted to the animal pole. This type of cleavage 

 has been observed following exposure: to urea, 2.34 per cent. 

 (Jenkinson, 1906, Figs. 38, 41); to NHJ (Jenkinson, 1906, Fig. 

 32); to Na 2 SO 4 (Jenkinson, 1906, Fig. 40); to temperatures 

 above 26 C. (Hertwig, 1895), et al. In my own experiments it 

 has been observed among eggs exposed six to ten hours to 2 

 per cent., 3 per cent, alcohol, a result that is not surprising when 

 one considers the solvent action of alcohol upon lipoid substances 

 generally. Four factors seem primarily concerned in the pro- 

 duction of these restricted cleavages, viz., time, concentration 

 or intensity of action of the agent used, physical effect of the 

 particular agent to which the eggs are exposed, and the stage 

 at which the eggs are exposed to the agent. So far as my 

 observations go, this type of cleavage, rare at best, is realized 

 only when the eggs are exposed to the inhibiting conditions at 

 the time of, or immediately preceding the appearance of the 

 first cleavage plane. If the eggs are exposed to concentrations 

 necessary to the production of this type of cleavage, several hours 

 before cleavage would normally begin, they never segment. The 

 physical effect of alcohol has been mentioned. The time element 

 is more or less obvious. Cleavage begins first, normally, in the 

 pigmented hemisphere, and under the conditions necessary for 

 the production of meroblastic cleavage the first cleavage plane 

 makes its appearance, in all probability, before the effect of the 

 inhibiting conditions (in the case of chemical substances par- 

 ticularly) penetrate the gelatinous membranes of the egg. 



B. Disturbances in Gastr illation. The various modifications 

 of the process of gastrulation, following inhibition, fall into several 

 more or less distinct groups, the particular types obtained de- 

 pending largely upon the severity of the inhibiting conditions 

 and upon the stage at which the eggs are introduced into the 

 experiment; i.e., depending upon the treatment and the physio- 

 logical condition of the eggs. Individual variation is an impor- 



