546 PROCEEDINGS OF THE ACADEMY OF [DeC, 



erwahnten Stammzellen ziehen sich in der That gegen den aboralen 

 Pol zu aiis, und schniiren schliesslich je eine Ideine Zelle ab, welche 

 unterdie IMesodermzellen [second and third quartets] zu liegen kommt." 

 These three cells Lang calls the upper endoderm, while the five cells 

 below he designates as the "mittler Entoderm." Goette (82) (p. 9) 

 also states that six or seven cells are budded into the interior of the 

 embrj^o of Stylochus as upper endoderm, but does not give further 

 information as to their exact origin. 



In Planocera inquilina, on the other hand, the three anterior cells of 

 the fourth quartet, viz., 4a, 46, and 4c, do not divide at this time nor 

 for a long time afterwards, if ever. The very large nuclei of these three 

 cells can be traced up until just before the formation of the alimentary 

 canal (fig. 36), and at this time they have not divided. The nuclei of 

 these cells, especially 46, become exceedingly large (fig. 32) and form- 

 excellent landmarks in the later stages. 



Wilson (98) finds and figures the bilateral division of 4tZ. He states 

 that the division of this cell into equal halves is an exception to the 

 rule, and that in about 90 per cent, of the eggs of Leptoplana the division 

 is markedly unequal (cf. his fig. 6, D, E and F). Wilson did not follow 

 the future division of these cells. He says (p. 22): "As regards the 

 iate of these cells, the inequality of 4d^ and 4(P [-id^-^ and M^-^] (often 

 very marked) is itself indirect evidence that they do not give rise to 

 symmetrical mesoblast bands as in the higher types, and I find no 

 evidence that either of them gives rise to mesoblast cells. Both seem to 

 have the same fate as the other entoblast cells, with which they exactly 

 agree in deutoplasmic structure, and enter into the formation of the 

 archenteron, as Lang has shown in the case of Discoccelis." 



It is peculiar that Lang should have observed 'cells budding into the 

 interior of the egg from three cells of the fourth quartet and not from 

 its other member, while I find that it is only this latter cell which 

 divides towards the interior, or in fact the only cell of this quartet that 

 divides at all. Lang gives figures of the spindles in the three anterior 

 cells of the fourth quartet in Discoccelis and also a detailed description 

 of these divisions. Although this work was done on living eggs, it 

 does not seem probable that so careful an observer would be mistaken 

 in the facts. We must conclude, then, that the three upper endoderm 

 cells of Discoccelis are in fact endoderm, and that this species differs 

 from Planocera in the division of the three anterior cells of the fourth 

 quartet. We would not be surprised to find such a coenogenetic 

 difference in different species of polyclads. Discoccelis perhaps shows 

 a more primitive condition in this respect, in that cells which are to 



