THE exogone^;. 239 



contained in vacuoles in the syncytium. This syncytial tissue with its 

 contained yolk-granules completely fills the interior o£ the bod}' and is 

 continuous externally with the body-wall ; there is at this stage neither 

 mesenteron nor coelom. 



The changes which result in the formation of these — the mesenteron and 

 coelom — take place in the last stages before the young worms become free. 

 In such a late larva (fig. 66) it is found that the syncytium has undergone a 

 profound modification. A cylindrical space has been formed through the 

 mass of yolk-containing tissue along the axis of the larva towards the dorsal 

 side. This becomes continuous with the lumen of the stomodasum in front 

 and that of the proctodeum behind. 



The most advanced larva3 observed have five seti»'erous segments. 



Exoyone verruyera and E. heterosetosa have the ova arranged exactly as 

 E. fustifera, in pairs from the 10th or 1 1th segment backwards. The stages 

 in the development are essentially the same, and the oldest fixed larva? have 

 each five setiserous segments. 



In Grubea quadrioculata the eggs, which are *12 mm. in long diameter, 

 are attached dorsally to the surface between the dorsal cirrus and the para- 

 podium — a pair on each segment from the 13th or 11th to about the 30th. 

 A peculiar feature of this form is that there are invariably two, sometimes 

 even three, more or less widely separated stages occurring together. The 

 arrangement of these is various, but usually all the eggs of the same stage 

 are grouped together; sometimes, however, there is more or less mixing. 

 The means of attachment are not very obvious; but from the position 

 occupied it seems highly probable that the secretion of the dorsal pedal 

 glands performs this function. 



The early stages are not in any noteworthy manner different from the 

 corresponding stages of E. fustifera. The early stages of segmentation were 

 observed and the formation of the ectodermal cap. A stage was obtained 

 in which the ectoderm as a thin, irregular layer encloses an endoderm of 

 about a dozen large clearly-defined cells. 



The chief peculiarities which appear in later stages are associated with the 

 fact that the embryo, instead of growing straight out, the egg-membrane 

 either disappearing or becoming converted into the cuticle, is developed 

 flexed dorsally within the egg-membrane (PL 18. fig. 68), from which it 

 only escapes when the tentacles and the parapodia of the primary segments 

 have appeared. 



As in Exoyone, the development of the embryo proceeds from two 

 separate centres — the dorsal plate and the ventral stomodrcal rudiment. The 

 former early becomes divided into two by a deep constriction, the anterior 

 part forming the head plate and the posterior the body plate. From the 

 latter the germinal bands (PI. 18. fig. 67) grow more directly downwards 

 (ventral), in accordance with the destined flexure. The head plate extends 



