312 EDWIN G. CONKLIN 



1. INTRODUCTION 



1. Historical 



The use of very strong centrifugal force in the study of the 

 structure and development of the egg was first made, I believe, by 

 Gurwitsch ('04) and Lyon ('06). Before that time the effects of 

 a rather weak centrifugal force of from 4 to 20 times gravity on 

 the development of eggs had been studied by Rauber ('84), 

 Roux ('84), 0. Hertwig ('99, '04), Morgan ('02), and Wetzel 

 ('04); but Gurwitsch used a very strong centrifugal force by 

 which he injured or destroyed the protoplasm for the purpose of 

 analyzing its structure, while Lyon used a force of from 4500 

 to 6400 times gravity in order to study its effects on develop- 

 ment. Lyon discovered that by means of this great force the 

 substances in the eggs of Arbacea, Asterias, Chaetopterus, 

 Phascolosoma, and Cynthia could be separated into three or four 

 layers differing in color or refractive index, and he made a brief 

 study of the development of the centrifuged eggs of Arbacea. 



This work was quickly followed by extensive studies of the de- 

 velopment of centrifuged eggs of Chaetopterus by Lilhe ('06, '09) 

 and of the centrifuged eggs of Arbacea, Cumingia, Cerebratulus, 

 Hydatina, the fish and the frog by Morgan ('07, '09, '10). Bo- 

 veri ('10) and Hogue ('10) studied the effects of strong centrif- 

 ugal force on the eggs of Ascaris; Conklin ('10) on the eggs of 

 Physa, Lymnaea and Planorbis; Konopacki ('11) on the eggs of 

 the frog, and Jenkinson ('14) also on the eggs of the frog. 



In general it has been found that yolk, which is usually the 

 heaviest substance in the egg, is thrown to the centrifugal pole, 

 oil or fatty substance to the centripetal pole, while the trans- 

 parent cytoplasm together with the nucleus occupies the middle 

 zone between the other two. Usually eggs develop normally 

 after this stratification, although the distribution of oil, yolk, 

 and pigment may be very abnormal; and even the cytoplasm 

 may be more abundant in certain cleavage cells than in normal 

 development, or less abundant in others, without permanently 

 interfering with typical development. 



