234 TORREY 



C). By this time their nuclei are mere dots of chromatin and 

 it is impossible to distinguish them from polar globules (Text- Fig. 

 lO, A), which, in fact, have an exactly similar fate. For a short 

 time they rest on the entoblast cells (Text-Fig. lo, C) and then 

 sink into them (Text-Fig. lo, Z^), gaining entrance by rupturing 

 the cell wall. I have seen as many as five inside one entoblast 

 cell, but they interfere in no way with its later divisions. Once 

 inside they are quickly absorbed and, in the fourteen hour em- 

 bryo, they have completely disappeared. This process of absorp- 

 tion consists first of the breaking down of their walls and then 

 the complete digestion of what is left of their cytoplasm and 

 nuclei (Text-Fig. lo, E). 



As far as I know, this is the first instance in which, beyond 

 all doubt, both the origin and the fate of rudimentary cells have 

 been determined, although similar cells have been described in 

 several other cases. E. B. Wilson ('92, '98) first called atten- 

 tion to them in his description of the very minute cells budded off 

 from the coelomesoblast pole-cells of Spio and Aricia. Again, 

 in Crepidula, according to Conklin ('97), the tip cells of the 

 anterior ends of the cross are very small and insignificant. He 

 believes that "in C. plana they are crowded entirely out of the 

 layer of the ectoblast cells and that they are thrown wholly 

 away." Miss Langenbeck ('98) describes two cells in the 

 blastocoel of the amphipod, Microdeutopus^ which without doubt 

 have sunk in from the surface layer. They soon begin to 

 degenerate and finally disappear. The observations of Tread- 

 well ('01), Mead ('97) and Child ('00) havealready been touched 

 on. In Asplanchna, according to Jennings ('96) two very small 

 cells or "vesicles" are budded off from the entoblast entirely 

 below the surface at the time of gastrulation. He was not 

 able to determine their fate, but thinks that the whole process is 

 comparable to the successive formation of polar bodies. Hacker 

 ('99) has grouped all such cells under the category of prepara- 

 tory or supernumerary divisions. " Their origin and fate," he 

 adds, "is a problem of the highest interest." 



The behavior of these cells in Thalasseina bears a remarkable 

 resemblance to that of the psedomesoblast (mesenchyme) cells 



