244 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Development of the Centrifuged Egg of the Frog.*— J. W. 

 Jenkinson notes that the egg of the frog has a polar structure — the 

 plasma and glycogen gradually decreasing in concentration from one 

 point on the surface towards the diametrically opposite point, while the 

 yolk gradually increases in concentration in the same direction. The 

 pigment lies in a superficial sheet in the plasmatic two-thirds of the egg. 

 As a result of fertilization this polar structure is replaced by a bilateral 

 structure when the grey crescent appears on one side of the egg at the 

 margin of the pigmented area. 



To this structure the embryo's ordinary development is definitely 

 related. The head of the embryo is formed near that pole towards 

 which the concentration of the plasma is increasing — the animal pole. 

 The blastopore closes near the vegetative pole, that towards which the 

 concentration of the yolk is increasing, while the dorsal side is that on 

 which the grey crescent appeared. 



By the centrifuge the materials of the egg are driven past one 

 another in opposite directions, the fat towards the centripetal (animal) 

 pole, with some entangled pigment and some plasma, the yolk and pig- 

 ment towards the centrifugal (vegetative) pole, while the movement of 

 the plasma is opposite at one end to that of the fat, at the other to that 

 of the yolk. 



Where the separation of materials is nearly or quite complete no 

 development is possible, nor even a normal cleavage. In less severely 

 centrifuged eggs there may be abnormal development. 



Yolk deprived of its plasma cannot divide, though nuclei may be 

 present. Meroblastic eggs may result. Derangement of material may 

 lead to degeneration of the front part of the head and the blastopore may 

 fail to close, the yolk plug remaining exposed. The first of these mal- 

 formations must be assigned to the excess of fat and lecithin, the defici- 

 ency of plasma and possibly of glycogen, and probably also of yolk, 

 since the yolk contains a store of nucleo-protein which is presumably 

 normally used in the production of fresh nuclear material. The per- 

 sistence of the yolk plug, on the other hand, is caused ultimately by a 

 deficiency of plasma. 



When the distribution of materials remains approximately normal in 

 the equatorial tract while altered at the poles, the development of the 

 middle part may be regular. With more centrifuging, however, the 

 usual distribution is lost in this region also, and then the power of 

 development disappears. The heart, pronephros, auditory vesicles, 

 nervous system, and alimentary canal all go, and an embryo results, in 

 which no development has occurred beyond the differentiation of some 

 mesoderm. 



A certain arrangement of most of the visible materials of the cyto- 

 plasm is a condition of normal development. The concentration of the 

 plasma, glycogen, and yolk must be graduated in a certain way, that of the 

 fat must be uniform. As Morgan pointed out, the position of the pig- 

 ment is alone unessential. But with this exception the factors to which 

 the egg owes its visible polar structure are also the causes of the pro- 

 duction from that egg of a normal organism. 



* Quart. Journ. Micr. Sci., lx. (1914) pp. 61-158 (6 pis. and 18 figs.). 



