EARLY DEVELOPMENT OF CM BR ELLA OCULATA. 379 



surface, the slight grooves separating the cells gradually becoming obliterated, until the 

 embryo appears merely as a sphere with a slightly roughened surface. In this 

 condition it remains for a considerable time, its general appearance differing little from 

 that of the unsegmented egg. Slowly, however, there appear on the surface certain 

 irregular but shallow grooves (fig. 4) which gradually extend from one part of the egg 

 to another. The process is tedious, and the appearance of the grooved embryos has so 

 strong a likeness to an unhealthy histolytic change that I invariably threw away my 

 developing embryos at this stage, feeling convinced that abnormal changes had ensued. 

 One set which were, however, kept over presented later a number of complete and 

 perfectly definite gastrulse (fig. 6), with large and gaping blastopore. Investigation of 

 sections, compared with similar stages derived from embryos preserved immediately 

 after catching, has convinced me that we are dealing with perfectly normal and 

 definite stages. The furrowed embryo (fig. 5) is an extremely common stage to find 

 in the brood-cavity of the parent. I have been enabled to recognise an exactly 

 similar stage in two species of Solaster. Fig. 16 shows the cells commencing to 

 arrange themselves in definite relation to the furrows, which are seen in section as 

 grooves. The cell-walls are becoming more distinct, and the peripheral cytoplasm 

 is uniting in definite directions into cytoplasmic tracts which pass inwards from 

 the furrows and divide up the cells into intermediate groups. From this stage on- 

 wards I have numerous transition stages to the condition shown in fig. 17, in which 

 the furrowed condition is at its maximum. The changes consist of a definite loss 

 of cell-walls and reduction of the whole embryo to a syncytial mass of vacuolated 

 cytoplasm and numerous nuclei. The peripheral cytoplasm of the cells has formed even 

 more distinct tracts leading outwards to the furrows, giving the appearance of a main 

 branch, the tract carrying numerous delicate lateral branches of cytoplasm passing out 

 between the nuclei, which still have yolk-granules arranged in definite radial relation to 

 them. These cytoplasmic tracts are commonly, as in the present instance, arranged 

 somewhat symmetrically, but the external view of the embryos shows that there is 

 considerable variation in this respect. In the centre of each cell-aggregate may be seen 

 a cavity in course of formation ; it is produced by the cells drawing away from each 

 other in all directions, and is usually intersected by thin plasmic strands. In a similar 

 way there is a central cavity produced by the separation of the cell-aggregates from 

 each other. This cavity increases in size, and eventually becomes continuous with the 

 cavities inside each cell-aggregate. Like the latter it is intersected by a faintly 

 staining plasma. Along the tracts there are numerous nuclei, which can be seen to be 

 more or less ranged in a row on each side of the tract. 



These are the structural facts ascertained by serial sections. Their interpretation 

 is open to question, and the conclusion to which I have been led may possibly be 

 erroneous. There is little question that rapid and important re-arrangement of the 

 cells (as represented in later stages by the nuclei) is taking place, and that in the 

 •last stage described the nuclei in the tracts are migrating either outwards or inwards. 



