266 W. G. YanName — Embryology of Eusti/lochus. 



plane than the others. They separate in the direction of a left- 

 handed spiral. The third cleavage is horizontal and the cells are 

 budded off in the direction of a right-handed spiral. The upper 

 four cells (about the animal pole) are quite conspicuously smaller 

 than the others and of about equal size in spite of the inequality of 

 the cells from which they are derived. The two, four, and eight- 

 cell stages are shown in Figs. 44, 45 and 46. 



Ego-s of Planocera nebulosa Verrill. 



&» 



In the eggs of Eustylochus I was unable to discover the sperm- 

 centrosome and sperm-aster. In the effort to find them many lots of 

 material, in every stage in which these structures might be looked 

 for, were preserved in fixing fluids of different kinds. Dozens of 

 slides were mounted, man}"- of them showing the matui'ation and 

 cleavage spindles in a beautiful state of preservation and satisfac- 

 toi'ily stained. These were gone over with the greatest care with 

 a high power objective, but I failed to find any trace of the sperm- 

 asters or anything that I had any reason to believe was the sperm- 

 centrosome. 



Still as the Gg^g of Eustylochus ellq^ticus is densely packed with 

 large yolk globules which not only stain with hsematoxylin but also 

 with plasma-stains, and are in many preparations so dark colored and 

 densely crowded that the sperm nucleus itself is often impossible to 

 distinguish, I was even then unwilling to accept this negative evi- 

 dence as final, and determined to examine the eggs of some other 

 planarian, where the conditions might be more favorable. 



Through the kindness of Dr. Wesley R. Coe of New Haven, to 

 whom I am also indebted for some preparations of the eggs of 

 Eustylochus which he had himself made some time previously, I 

 received during the spring of this year (1899) some living individu- 

 als of the rather rare species Planocera nebulosa Girard. These he 

 obtained while collecting the large nemertean Cerebratulus lacteus 

 Verrill, in the tubes or burrows of which this species usually lives, 

 though it is occasionally found elsewhere. It can be kept in confine- 

 ment more readily and lays even more freely than Eustylochus ellij)- 

 ticus. My specimens laid their eggs in March and April, more 

 abundantly during the latter month. 



These specimens of Planocera nebulosa were pronounced by Prof. 

 Verrill identical with those he has called by this name in his work 

 on the marine planarians of New England (29). In that work (p. 



