Miscellaneous. 407 



there is a distinct and prominent septum in each valve ; and the 

 laminar ridge in the smaller valve is much slighter, and is inter- 

 rupted by the septum to which it is attached. Both species occur 

 together on the English and Irish coasts, and at Etretat in Nor- 

 mandy ; and A. capsula was recorded by the late Prof. Sars as fossil 

 at Kirkoen, near Christiania. 



Notes on the Early Stages of some Poli/chcetous Annelides. 

 By" E. B. Wilson. * 



In view of the morphological interest of the marine annelides as 

 the most highly specialized forms among the " Vermes," and the 

 scarcity of detailed accounts of their early stages of development, 

 the following preliminary abstract of studies on the eggs of Areni- 

 cola and Clymenella seems of some interest. The eggs are small 

 and very numerous, and are imbedded in transparent gelatinous 

 masses issuing from the mouths of the tubes or burrows inhabited 

 by the worms. The egg-masses of Arenicola are of great size, 

 being sometimes 5 or 6 feet in length and from 2 to 4 inches in 

 diameter r such a mass must contain several hundred thousand 

 eggs. Those of Clymenella are usually about the size and shape 

 of a pigeon's egg ; the eggs are much fewer and considerably 

 larger than those of Arenicola. 



The whole course of development is essentially alike in the two 

 forms. No polar globules of constant relation to the yolk were 

 observed. The first cleavage divides the egg into two unequal 

 spherules. The second, passing at right angles to the first, divides 

 the smaller spherule into two equal parts, and the larger into two 

 unequal parts. The third cleavage separates from these four 

 blastomeres four much smaller ones at one pole of the egg. The 

 latter (micromeres) soon become so displaced as to alternate with 

 the former (macromeres). The micromeres now divide more rapidly 

 than the macromeres, which they come ultimately to include by 

 growing down over them. The ectoderm is formed by the deriva- 

 tives of the micromeres, and in part, I believe, of the macromeres. 

 The remaining portions of the macromeres form the entoderm. 

 Two large spherules, which originally formed a part of the largest 

 of the four primary blastomeres, are visihle up to a late stage at the 

 posterior extremity of the embryo. They are at first at the surface, 

 but ultimately are grown over by the ectoderm and disappear. It 

 is possible that they are concerned in the formation of the mesoderm 

 and are to be regarded as primary mesoblasts. The mouth arises on 

 the ventral side nearly opposite that pole of the egg where the first 

 four micromeres were formed. The anus arises at the posterior end 

 of the embryo. The egg-membrane is directly converted into the 

 cuticle of the larva. The egg exhibits, during segmentation, alter- 

 nate periods of activity and quiescence. 



The embryo acquires two dorsal eye-specks, praeoral and prae- 



