Habits and Earl\ Development of Lincrgcs mercurius. 165 



Centrifugalizcd eggs. It has been found by Lyon (1906), Lillie (1906), 

 and Morgan and Lyon (1907), as well as by myself that the substances of 

 the eggs of many animals may be separated into zones by means of strong 

 centrifugal force. When the eggs of Linerges are centrifuged immediately 

 after being laid, but little separation of the egg-substances is produced even 

 if they be centrifuged at the relatively rapid rate of I2,(X)0 revolutions per 

 minute for 2 minutes. In later stages (just before and after the first cleav- 

 age) the substances of the tg^ separate much more readily. 



Fertilized eggs in the stages just before and after the first cleavage 

 when centrifuged for i minute at the rate of 10,000 revolutions per minute 

 remain unaltered in structure and subsequently develop normally. If the 

 same eggs are centrifuged at the same rate for 2 minutes the substances 

 of the egg are separated into a clear, a blue, and a yellow zone, of which 

 the first is the lightest and the last the heaviest. Many such eggs are 

 evidently killed or so injured that they do not develop further, one such being 

 shown in figure 38 : others undergo irregular cleavages, which show abnor- 

 mal distribution of these ooplasmic substances. In some cases irregular 

 planulas are formed, with an abnormal distribution of the egg-substances, 

 and these may live and swim about for at least 24 hours. For example, in 

 one case the clear substances formed a prominence on one side of the larva, 

 while the remainder of the planula was blue or bluish-yellow. Whether 

 such a planula would give rise to a normal scyphistoma was not determined, 

 since the larvae were not reared to this stage. And this leads me to remark 

 that in an organism in which there are so few differentiated parts as in a 

 planula it is practically impossible to determine with certainty whether ex- 

 l)eriments on the egg have modified its potency ; only the study of later and 

 more complicated stages could yield conclusive evidence on this point. 



CONCLUSIONS. 



The organization of the egg of Linerges. The differentiations of the 

 egg are limited to polarity and to the existence of concentric layers of 

 difl^erentiated ooplasm. Polarity is clearly marked in the egg before matura- 

 tion, and as in practically all other animals the maturation pole becomes 

 the aboral or ectodermal pole of the gastrula and planula, while the opposite 

 pole of the egg becomes the oral or endodermal pole. 



The egg and embryo are radially symmetrical. The cleavages are ap- 

 proximately equal and synchronous, and the earliest dififerentiation of the 

 cleavage cells consists in the appearance of the rounded endoderm cells 

 at the vegetal pole, while the ectoderm cells are long and narrow, form- 

 ing a columnar epithelium (figs. 31-34). 



Of the different concentric layers of ooplasm, the peripheral one is 

 nearly free from yolk, and is slightly thicker at the animal pole than else- 

 where. It becomes the peripheral layer of the blastula and gastrula and 

 gives rise to the cilia which clothe the ectoderm. 



