1907.] NATURAL SCIENCES OF PHILADELPHIA. 545 



the very small aiuoiint of cytoplasm seems to me to preclude such a 

 fate ill Planoccra. I cannot find or at least cannot recognize these 

 cells after the ectoderm has overgrown the lower pole of the egg, and so 

 1 am unable to follow tlieir later history. Text fig. 3 gives an accurate 

 representation of the condition of the nuclei of these cells at that stage, 

 which is about as late as I am able to follow them. These nuclei have 

 shown this same condensed condition of the chromatin almost from 

 the time of their formation (cf. figs. 17, 19, 20, 25, 26, 27). These 

 cells are proljably carried in with the invaginating pharynx and then 

 absorbed by the endoderm. I have sometimes found what I thought 

 were remains of these cells, but of this I cannot be certain. 



Discussion. 



We may now return to a comparison of the observations of previous 

 students with regard to the foin-th quartet, and especially Ad, in other 

 polyclad eggs. As has been already noted (p. 536) the peculiar bilateral 

 division of the posterior cell, Ad, has been known since the work of 

 Hallez (79). Hallez believed that this posterior cell divided without 

 an amphiaster, and regarded the product as not equivalent to the other 

 four cells, but in the nature of cell sap. Goette (82) states that in some 

 cases the cells 4a and 4c of our nomenclature also divide, so that there 

 are seven cells, on the surface of the embryo, formed from this quartet. 

 This happened£only in some of the eggs of Stylochus (Stylochopis) 

 pillidinm. In other eggs of the same species only the posterior cell 

 divided. In such cases Goette foimd that this cell often divided twice, 

 forming four cells, all on the surface of the egg. 



Lang (84) finds that in Discoccelis all the cells of this fourth quartet 

 divide at about the same time, but in different directions. The pos- 

 terior cell divides horizontally as described, while each of the other 

 three^cells buds into the interior of the egg a cell which he calls upper 

 endoderm. Lang says (p. 337) : "Es treten in ihnen [fourth quartet of 

 our system] Richtungsspindeln auf, und zwar wieder in der oft ange- 

 flihrten Reihenfolge. Die Richtungsspindel der grossten Stammzelle 

 des Entoderms [4a] verliingert sich excentrisch in der peripherischen 

 Verlangerung der Ebene, welche man sich durch diese Stammzelle und 

 die Hauptachse des Eies gelegt denken kann, und welche der jMediane- 

 bene entspricht." .... (P. 338): "Umnittelbar befor sich die grosste 

 Entodermstammzelle [Ad] in ihre zwei seitlichen Hiilften getheilt hat, 

 zeigen sich audi in den drei Uebrigen, Richtungsspindeln, die aber eine 

 ganz and ere Direction haben. Sie liegen niimlich parallel zur Haup- 

 tachse, d. h. sie zeigen eine dorsoventrale Richtung. Die drei 



