440 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCUUM. 



would meet with success. The studies should be carried on, whether in Brittany or 

 at Naples, during the spring or early summer. I have been unable to verify the obser- 

 vations of Hatschek ('83) upon Sipunculus except as regards a single stage in the 

 larval development of S. tessellatus Kef. taken in the tow at Naples. 



I wish here to make hearty acknowledgment to all who have aided me in various 

 ways in carrying on these studies, especially to Mr. Alexander Agassiz and likewise 

 to Dr. Mark, whose interest in the work and helpful advice have been of great value 

 to me. I am indebted also to the members of the respective staffs of the Zoological 

 stations at Roscoff and at Naples, to Professor Fabre-Domergue, and not least to 

 Professor Korschelt, who generously extended to me, while in Europe, the privileges 

 of his laboratory at Marburg. 



The present paper deals especially with those facts in the development of Phas- 

 colosoma which throw light upon the nature of the embryonal envelope in Sipunculus, 

 and hence is limited in scope to a consideration of the ectoderm of the trochophore. 

 A more complete account of the embryology of Phascolosoma will soon be published. 



II. THE EMBRYONAL ENVELOPE IN SIPUNCULUS. 



To make clear the nature of the embryonal envelope of Sipunculus, it will be well 

 to remind the reader of the main features of the development of that form as described 

 by Hatschek ('83). After a cleavage in which the blastomeres are of nearly equal 

 size, the slightly larger cells at the vegetative pole become invaginated, and the 

 embryo assumes the shape of a gastrula which has the more essential features of a 

 trochophore (PI. XXXII, Fig. 1). There is an apical plate which bears long cilia, 

 and a circular band, composed of two or three rows of ciliated cells, corresponding 

 in position to a prototroch. At the posterior pole is the invagination of endoderm, 

 and a pair of mesoderm pole-cells project from the dorsal lip of the blastopore into 

 the well-marked segmentation-cavity. 



At this stage not only do the cells at the vegetative pole become separated from 

 the zona radiata, but at the active pole the marginal cells of the apical area, which 

 surround the four characteristic rosette cells, become separated from the zona radiata 

 and sink, forming a deep ring-shaped furrow — the amniotic cavity of the head (PI. 

 XXXII, Figs. 1-5, cav. am. ce.). 



Thereupon the closure of the blastopore ensues by a growth ventrad and forward 

 of the ectoderm of its dorsal lip, at which point the mesoderm cells are situated (Figs.- 

 2, 3, 4). This process results in the formation of a median somatic plate (Rumpf- 



