1907.] NATURAL SCIENCES OF I'lIILADELPIIIA. 517 



tMidoderni cell, as Goette had found. He dcii\('<l tlic uliiiientary canal 

 (i\nn the upper and lower endoderni cells, while the niacronieres (middle 

 <'nd()(UM-ni) broke up into food yolk and were absorbed by the other 

 cells. All of the last four writers found that the posterior niacroniere 

 behaved differently from the otlier three in that it divided with its 

 luiclear s])indle lying horizontally, thus giving rise to five macromeres 

 (foiu'th (piartet). 



IMatkuial and Mktiiods. 



The follow iiiii' paper is based u])on the study of a species of polyclad, 

 rianocira inquilina, descril)ed l)y Prof. "Wheeler in 1894. The material 

 was obtained at the Marine Biological Laboratory at Woods Hole, 

 ^Massachusetts, during the months of July and August of 1906. 



Planoccra inquilina is i)eculiar among polyclads in that it leads an 

 apparently parasitic life. These worms are found in the branchial 

 (•haml)er of the large whelk, Sycotypus canaliculatiis Gill. As Wheeler 

 suggests, it seems probal)le that they live on the excretory or waste 

 products of this gasteropod. No evidences that they feed on the 

 tissues of the host have been found. The adult worms were obtained 

 in considerable abundance, averaging about three worms for every 

 Avhelk opened. The adult pol3'clads were transferred to dishes of sea 

 water, in which the water was changed by means of a system of balanced 

 syphons. These syphons served to keep the water free from sand and 

 dirt, and also prevented the overflow of the water and the escape of the 

 worms. The animals usually laitl eggs soon after being brought into 

 the laboratory. As described by Wheeler, the eggs are laid in spiral, 

 g;elatinous capsules containing anywhere from 100 to 2,000 eggs each. 

 Each egg is surrounded by a separate egg membrane and the whole is 

 imbetlded in tlie capsule material. In many cases two eggs are 

 deposited in a single egg membrane, both of which develop normally. 

 This is the usual way in which polyclad eggs are deposited. The tough 

 capsule is difficult to penetrate with fixing and staining reagents. 

 This fact no doubt is one of the chief reasons why so few embryologists 

 have worked on polyclad development. The eggs of Planocera 

 inquilina seem more favorable in this respect, and with care it is possible 

 to get very good preparations. The egg capsules were deposited 

 against the sides of the dishes, and it w'as necessary to cut them away 

 with a scalpel. 



WTieeler (94) did not succeed in getting the eggs of this species to 

 develop under laboratory conditions. On the contrary I experienced 

 no difHcultv of this kind. Stages from the maturation to the free 



