222 NIKOLAS KLEINENBERG. 



Now, what is the origin of the cells of the mesoderm ? 

 According to Kowalewsky, the two large cells produce the 

 middle layer in L. ruhellus, while in L. agricola, where 

 such cells do not exist, the well-developed endoderm pro- 

 bably furnishes the material for the formation of the meso- 

 derm. In Euaxes the middle layer is derived directly 

 from the division of the four first blastomeres.^ Hatschek 

 affirms still more decisively that in L. ruhellus the mesoderm 

 is derived from the two large cells. ^ 



There is no doubt that the mesodermic arches begin with 

 the appearance of the few small cells at the sides of the 

 mesoblast cells, and that their development proceeds from here 

 towards the opposite extremity; this is certainly a remarkable 

 fact, but is it enough to enable us to decide the part which 

 the large cells play in the formation of the mesoderm ? I 

 have not met with states of incomplete division in the latter, 

 but very little value can be attributed to such a negative 

 result, especially because it is matter of general experience 

 that, in rapidly growing tissues, cells in which the process of 

 division has really begun without being completed are rarely 

 observed. This fact may be explained by the rapidity of the 

 process of fission, after the previous internal changes have 

 been effected. But the observation that the large cells retain 

 their volume apparently unaltered from the beginning to the 

 end of the embryonic life may raise more serious doubts as 

 to their reproductive activity. 



At least I have not been able to make sure of the exist- 

 ence of oscillations in the size of the large cells which would 

 have justified the supposition that they deprived themselves 

 of a portion of their substance to give rise to the cells of 

 the mesoderm. Kowalewsky represents a stage in which 

 each of the large cells is divided into three smaller ones of 

 nearly equal size.^ In L. trapezoides this never happens ; on 

 the contrary, the cells of the mesoderm, which are in 

 contact with the large cells, are always among the smallest 

 and most compressed. Notwithstanding all this I am also 

 of opinion that there must be a production of new cells 

 from the two large ones, solely because they show very 

 often the phenomena which may be considered with great 

 probability as a necessary preparation antecedent to the 

 formation of new cells. In this case the mode of reproduc- 

 tion would be what is ordinary called gemmation ', cells 

 greatly inferior in size to the mother-cell would separate 

 themselves from a point of the surface, and the mother-cell 



> Loc. cit., pp. 16, 23, 29. 



2 ' Zeit. fur Wiss. Zool./ T. xxix, 1S77, p. 5i5. 



^ Loc. cit., pi. vi, fig. 14. 



