442 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULID^. 



will be sufficient to point out in conclusion that, by the growth of the somatic plate 

 chiefly from the dorsal side forward and laterally, the definitive surface of the young 

 larva is completed. The mid-dorsal double row of cells first disappears, possibly to 

 form a part of the serosa. The broad dorsal part of the somatic plate then begins 

 to extend forward and laterally, becoming thinner than the ventral, the lateral line 

 of union of the two being situated far toward the ventral side. It should be noted 

 in passing that in its growth the somatic layer of mesoderm outstrips the ectodermal 

 somatic plate, so that for a time even after the ccelom is established, the serosa forms 

 the ectodermal covering of the body proper, beneath which lie the two layers of meso- 

 derm enclosing the ccelom and beneath them the endoderm (Fig. 6), a stage corre- 

 sponding exactly to the trochophore of Phascolosoma before shedding the egg- 

 membrane. 



III. THE PROTOTROCH OF PHASCOLOSMA. 



In a form so closely related to Sipunculus as Phascolosoma we should expect 

 to find a similarity in the main features of development, and it has seemed very 

 remarkable to me that a structure of such prominence as the embryonal envelope 

 or serosa of Sipunculus should not have its homologue in Phascolosoma. 



I shall endeavor to make it clear that in Phascolosoma not only the serosa is 

 represented, but that the rest of the ectoderm is disposed in an essentially similar 

 manner to that in the Sipunculus embryo, the most obvious differences being those 

 due to the presence in the egg of Phascolosoma of a much larger amount of yolk than 

 in that of Sipunculus. The two species of Phascolosoma to which I have given most 

 attention, namely, P. gouldii Diesing of the American coast and P. vulgare Blainv. 

 of the British Channel, differ from each other in their development in slight details; 

 the egg and trochophore of the former are more opaque than in the European form, 

 owing to a difference in the amount of yolk. 



The cleavage in Phascolosoma is very unequal. The egg like that of certain of 

 the nemerteans (Micrura cseca, Cerebratulus leidyi, and C. lacteus) is remarkable for 

 the large size of the first set of micromeres and their descendants; these "micro- 

 meres" in the eight-cell stage slightly exceed in size the macromeres, except in 

 quadrant D. Thus a preponderance of yolk is located in the "active" half of the 

 egg; and the prototroch cells of the trochophore, which arise from this half, are laden 

 with large yolk-granules coarser and more abundant than those of the endoderm. 



In the 48-cell stage of P. vulgare at the age of about ten hours, cilia begin to 



