EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 387 



Before leaving this stage we should emphasise the fact that internal asymmetry is 

 clearly commencing, evinced chiefly in the predominant growth of the left lateral coelom, 

 and the forward extension of the left posterior ccelom in marked contrast to the right. 

 The total result is that the organs belonging to the left side fill up more than half 

 the larva and throw the median central ccelom to the right. 



Stage D — (figs. 46-76). — Stage D is the culminating point of the larval entity, 

 as already described. Externally it is perfectly symmetrical and its orientation is 

 easy. The anterior end is indicated by the trilobed pre-oral lobe, the dorsal surface by 

 the single largest of the processes, and the lateral surfaces by the flattening from side to 

 side which soon becomes evident (figs. 71-76). It is the internal structure of this stage 

 which must be relied upon to give indications of the structure of the hypothetical 

 bilateral ancestor of the Echinodermata. 



A general idea of the internal structure may be derived from an inspection of 

 1 cleared ' specimens. Fig. 50 shows a dorsal view of one, and the large and spacious 

 pre-oral ccelom, throwing processes into the three parts of the pre-oral lobe, can be 

 recognised. The anterior coelom is now constricted into its four elements of pre-oral 

 coelom, central coelom, and two lateral cceloms, which all still communicate by small 

 passages. On the right side can be recognised the central ccelom, and dorsal to it is 

 the commencement of the pore-canal (p.c) growing out as a pocket towards the right 

 side of the larva. Late stages of D often show the canal freely opening to the exterior 

 in the centre of the right side. In the centre of the larval body is the dark mass of the 

 mesenteron, covered by the dorsal horn of the left posterior ccelom (D.L. 3). Fig. 51 is 

 a ventral view, and here we note the presence of the right posterior ccelom (R. 3), small 

 but distinct, and lying internally and ventrally to the right lateral ccelom. In the 

 lateral view (fig. 52) the openings of mesenteron into anterior and posterior cceloms 

 respectively can be recognised ; the full extent of the left posterior ccelom and the stone- 

 canal, as a groove along the posterior border of the anterior ccelom, are to be noticed. 



Hence at this stage all the cavities of the body are in free communication. The 

 pre-oral ccelom has connected with it the three pre-oral processes, the two lateral 

 coeloms, the central ccelom and the pore-canal, while the posterior ccelom has its left 

 portion with dorsal and ventral horns, and its right portion with its ventral horn only. 



It is now not difficult to follow the series of sections. Figs. 58 to 62 illustrate 

 a transverse series taken through a sj)ecinien which might be termed C/D or a very early 

 D. It can be compared, section for section, with the series (figs. 38-45) of stage C. 

 In fig. 58 we see that the pre-oral ccelom shows indications not only of the two ventro- 

 lateral pre-oral processes, but a pair of grooves which will later form processes into the 

 divided median process seen in stage E. In fig. 59 the opening of the pre-oral ccelom 

 into the mesenteron is seen towards the left, as in fig. 40. The central ccelom is not 

 much different from stage C, but the left lateral ccelom has increased in size, spreading 

 out dorsally and ventrally, and pushing the left posterior coelom over toward 

 the right. 



