386 DR ARTHUR T. MASTERMAN ON THE 



a trace of the blastopore as an epiblastic thickening, occasionally with a short lumen. 

 It is important to notice that the ' anlage ' of the central ccelom is median, for although 

 it is in fig. 31 very slightly to the right, the fact that the epiblastic pit (fig. 34) also 

 appears slightly to the right, which is not actually the case in a properly orientated 

 larva, justifies us in this opinion. Fig. 35 is a median coronal section of a late 

 B stage ; in it the mesenteron is seen constricting off posteriorly, and the elongated 

 anterior ccelom is evident. 



Stage C is an important epoch in the larval history. In it there appear the first 

 indications of asymmetry. Figs. 36 and 37 are dorsal and ventral views respectively 

 of the entire larva viewed as a transparent object after being cleared in oil of cloves. 

 Such views are difficult to obtain owing to the thickness of the epiblast, and some 

 amount of optical distortion is inevitable, but if they be carefully compared with a 

 series of transverse sections (figs. 38-45) a clear idea is easily obtained. The anterior 

 ccelom has increased greatly forwards and backwards. At its anterior end it is assuming 

 a triangular shape preparatory to trifurcating into the pre-oral processes not yet 

 developed (fig. 24). Posteriorly its two lateral cceloms (L. 2, E. 2) have grown back- 

 wards very rapidly, the right one to the level of the posterior border of the mesenteron 

 or thereabouts (figs. 36 and 53), and the left one still further (figs. 36 and 45) ; this 

 more extensive growth of the left lateral ccelom has dragged the opening of the 

 mesenteron into the anterior ccelom downwards to the left (fig. 40), a character shown 

 in every specimen examined. Not only has the left lateral ccelom extended further 

 backwards, but it is also clearly seen in the sections to be of larger size. The posterior 

 wall of the anterior ccelom still extends slightly dorsally over the mesenteron, but the 

 central ccelom has now commenced to travel to the right, its distal end lying over the 

 right lateral ccelom (fig. 42). But the most marked asymmetry is produced by the 

 posterior ccelom. Its left portion has grown forwards dorsally and ventrally in a pair of 

 long horns which lie almost exactly in the median line. The dorsal horn (D.L. 3) is the 

 shorter and runs forward over the mesenteron to about the level of the central ccelom, 

 nearly up to the posterior wall of the anterior ccelom. It appears to actually squeeze 

 the central ccelom to the right (fig. 36). The ventral horn (V.L. 3) passes forwards 

 below the mesenteron (figs. 37 and 40) till it touches the ventral wall of the anterior 

 ccelom. Still it pushes forwards between the epiblast and the latter, till it extends over 

 the middle line of the larva. The rapid growth of this left posterior ccelom disguises 

 the right posterior ccelom altogether, which may, however, still be recognised (fig. 36) 

 as a small process (R. 3). Thus the asymmetry of the posterior ccelom is even more 

 marked than that of the anterior. 



Of histological differentiation there is little to note. The mesenteron is becoming 

 sharply defined by its thicker walls, whilst the ccelomic walls do not materially differ 

 from each other. The hy pen chyme varies greatly; in some cases it is still quite intact, 

 but in many it is seen to be rapidly breaking up into isolated fragments. The specimen 

 figured was almost freed from its presence, but this is somewhat exceptional. 





