On the Life-History and Development of the Genus Myzostoma. 553 



meut (fig. 4) . A period of repose tlien ensues. Fifteen minutes after 

 the first divisiou, the larger of the two Segments divides and once more 

 in such a manner that one of the resulting segments greatly exceeds 

 the other in size (fig. 5). Thus there are now three segments all of 

 unequal size , viz. a small segment as the result of the first divisiou. a 

 larger segment and one double the size of this, the latter two formed in 

 the second division. Again there is a pause about a quarter of an hour 

 and then the rhythmic act of segmentation proceeds. In the next division 

 the smaller of the two last formed segments is alone concerned. It 

 divides into two pretty equal halves. At this stage (fig. 6) the segment- 

 ing egg consists of three pretty equal small spheres ; the future epi- 

 blast, and one large hypoblastic sphere. the latter containing the yolk, 

 and also the yolk-nucleus, whieh at this stage is still visible. 



The succeeding stages vary somewhat, tho' the final result is the 

 same. Indeed here we have variations similar to those recorded by 

 Wilson ^ for Retiilla. The variations here are not so numerous, indeed 

 they are ouly two in number. The first mode is a successive division of 

 the epiblastic spheres commencing with the one first formed, that is, with 

 the smaller of the two first formed segments. Its division is followed by 

 that of the other two epiblastic spheres, one after the other (figs. 7, 8, 9) . 



In the second mode the whole three of these epiblastic segments 

 may divide simultaneously (fig. 9). In both cases we get as the result 

 one large hypoblastic segment, and six small epiblastic segments. which 

 form a hood or cap over the former (fig. 9) . The hypoblastic segment 

 now divides iuto two equal spheres (fig. 10), in some few cases this di- 

 vision only takes place after some of the epiblast spheres have again 

 divided (fig. 11). The epiblastic spheres now ali divide and in doiug 

 so grow more and more over the hypoblastic cells. The division goes 

 rapidly on, but is very difficult to follow in such small elements. It 

 results in the enclosure of the hypoblast cells within a layer of epiblast 

 cells. The point where the epiblast cells meet, viz. the blastopore, closes, 

 but on its site is formed the future mouth. A stage just before the 

 closure of the blastopore is shown in optical section drawn under Vis 

 in. Leitz Homogeneous Immersion in fig. 13. Here the layer of epiblast 

 cells encloses four hypoblast cells, and two mesoblast cells, the latter 

 being darkly coloured for the sake of distinction. Ali these six cells 

 have been formed by the di vision of the two hypoblast cells. 



1 E. B. Wilson, Variation in the yolk-cleavage of Reìiilla. Zool. Anz. 1882. 

 p. 545—548. 



