EMBRYOLOGY. 1 



On the Development of Nereis dumerilii.- — Though this is 

 but the first part of a contribution to the development of Annelids, to 

 be followed by an account of the formation of the organs, yet the 

 untimely death of the author makes it almost necessary to regard the 

 present contribution as complete. 



The very interesting series of forms discovered by Claparede in this 

 single species, N dumerilii, have been again studied by the present 

 author, who believes their interrelations to be the following : 



A small Nereis 12-15 mm. long in an immature state may have 

 two quite different fates. It maf transform in September and Octo- 

 ber into a pelagic Heteronereis, becoming sexually mature in Febru- 

 ary and March, and depositing pelagic eggs that contain little yolk 

 and develop with a metamorphosis ; it may grow to a length of 

 15-30 mm. and then become sexually mature as a Nereis laying eggs 

 in tubes in April to July, which eggs contain much yolk and develop 

 without a metamorphosis ; or still growing, in June and July become a 

 Heteronereis 55-65 mm. long that lays eggs in tube, with little yolk 

 and probably developing with a metamorphosis. 



There are thus two Heteronereis states, a little one with pelagic hab- 

 its and metamorphosis and a large one without any pelagic life, and 

 unknown, probably, indirect development. There are two Nereis 

 states, one small and immature, the other large and either on the road 

 to the large Heteronereis state or becoming a sexual Nereis with a 

 direct development. It is this last large sexual Nereis that is treated 

 of in the present article. 



Obviously much yet remains to be done here by one having control 

 of aquaria for long periods, as it is still unknown what may be the 

 causes leading to the acquirement of these various states or whether 

 there is any regular sequence or alternation in their occurrence. This 

 could be done upon the common, equally polymorphic, Nereis of our 



In all forms of Nereis dumerilii the sexes are separate. In the 

 large Nereis state studied, the females readily lay the eggs in transpar- 

 ent tubes (secreted by the parapodial and other epidermal glands) 



Edited by Dr. T. H. Morgan, Bryn Mawr, Pa. 

 Pi*? r '7 C ' V ' Wistin g hausen in Mitth. Zool. Sta. Neapcl. 10, July, 1891, pp. 41-72- 



