EMBRYOLOGY. 247 



The component cells at this time are comparatively large, and as they are 

 filled with yolk granules they are clearly distinguished from the stomodeal 

 elements. In the latest represented stage (Plate 14, fig. 4) the mid-gut in its 

 central portion appears as a confused mass of cells owing apparently to great 

 irregularities in the position of the component cells. The general outhne of 

 the organ, it is beheved, is correctly indicated. No trace of a proctodaeum 

 or cloacal chamber has been discovered. 



Comparisons. — The only other accounts of the development of a Soleno- 

 gastre comes from the hand of Pruvot who investigated two species, Myzomenia 

 banyulensis and Proneomenia aglaopheniae. The two reports are very brief, 

 and in some respects the observations coincide with those of the foregoing 

 paragraphs; on the other hand there are certain statements that are open 

 to question. These agreements and differences will now be considered in 

 brief. 



In Myzomenia the eggs as they leave the gonad and enter the coelomoducts 

 are naked. A membrane is present when the eggs leave the body, and it there- 

 fore follows that the shell gland forms the envelope. It does not follow, how- 

 ever, that the ducts are not at the same time excretory organs though this 

 may indeed be the case. In Halomenia the ova in the gonad are surrounded 

 by envelopes of the same character as surround the eggs stored in the cloacal 

 chamber. The shell gland is highly developed in this species, but what part 

 it plays in the reproductive process is not clear. 



The four-cell stage comprises one large and three small cells; successive 

 divisions of the smaller cells enclose the products of the larger. At a later 

 period the embryo becomes cap-shaped and a large pit in the ventral half of 

 the embryo is believed to represent the blastopore. The test now becomes 

 clearly defined, an apical tuft of cilia arises, the trunk region, button-like in 

 form, protrudes beyond the margin of the test and a ciliated terminal ring 

 encloses a pit-like depression termed the blastopore. Calcareous plates arise 

 at various points upon the surface of the body, whereupon the metamorphosis 

 ensues. 



Turning now to the development of Proneomenia it is evident that in 

 several respects it bears a close resemblance to Halomenia. The blastomeres 

 are described as being shghtly unequal, surrounding a small blastocele. Invagi- 

 nation occurs and a large blastopore is formed. The larva now elongates and, 

 covered with a ciliated coat borne on five tiers of cells forming three zones or 

 segments, the resemblance to the Myzomenia larva is fairly complete. There 



