264 W. G. VanJVame — Embryology of Eustylochus. 



the development of the planarian cgs^ into line with the observations 

 on those of animals of other classes. 



But as I have been so fortunate as to obtain some unusually- 

 good preparations of polyclad eggs, a description of them may be 

 of interest not only on account of the additional light it may throw 

 on the points left unsettled or passed over briefly by the previous 

 writers, but also as confirming many of their conclusions by means 

 of investigations on the eggs of different genera and species. 



I studied at first only the eggs of Eustylochits ellipticus (Girard) 

 Verrill, a polyclad found abundantly in the vicinity of New Haven, 

 Conn., during the winter and spring months, on the beach a little 

 below high water mark, clinging to the under side of large stones. 

 The animals may be kept alive for many Aveeks in sea-water, if it is 

 occasionally changed. A few eggs were laid in January and Febru- 

 ary, but more were produced during March, April and May. 



Girard (7, 8) published an account of the embryology of this spe- 

 cies under the name Planocera elliptica, nearly half a century ago. 

 His figures are remarkably well drawn, and considering that he wrote 

 at a time when the maturation, and the significance of the fertiliza- 

 tion and cleavage of the eg^ were not understood, the number of 

 facts correctly observed by him is remarkable. No one else, so far 

 as I am aware, has published anything in regard to its embryology, 

 and of course not only the cytology, but much else remained as a 

 new field for investigation. 



The eggs are laid in clusters or sheets containing from one to two 

 dozens up to several hundreds, arranged in a single layer and closely 

 attached together with a white mucus-like secretion, which is at 

 first very sticky and adheres to everything it touches, but gradually 

 becomes harder. The eggs lie so close together in the cluster that 

 the Qgg membranes, though not the egg itself (for there is a space 

 between them), become more or less polj^gonal and no definite 

 arrangement of the eggs can usually be made out. In Planocera 

 nebxilosa Girai'd, another polyclad of which I afterwai'ds obtained 

 eggs, they are not placed so close together, and their arrangement in 

 a zig-zag line extending back and forth across the cluster, can readily 

 be seen. It took an individual of this latter species less than a min- 

 ute to lay a row of eggs across a cluster about three-sixteenths of 

 an inch wide and back again, the animal keeping the forward end of 

 the body stationary or advancing it slightly as row after row of eggs 

 was laid, and slowly moving from side to side the posterior part of 

 the body, where the reproductive openings are situated. In the case 



