1260 
the closing of the cerebral plate as the overgrowth of the lateral 
ridges. 
There is yet another circumstance I should like to emphasize. 
Not only the ectoderm of the cerebral plate but also that which is 
situated in front of the transverse cerebral fold and which according 
to my theory is equivalent to that part of the apical plate of the 
Annelid trochophore which in Craniotes is not incorporated into the 
cerebral plate, is considerably thickened, and as for example in 
fig. 1 (pr. cer.) it exhibits an equally clear separation between the 
upper and lower layers of the ectoderm as the cerebral plate. Also 
in fig. 2 this agreement between cerebral plate’ and the part of the 
apical plate in front of it, which we might call the praecerebral 
part is evident. In the course of further development, however, a 
difference between the two parts of the apical plate evidences itself. 
In the cerebral, just as in the medullary plate, an intimate union 
of the upper and lower layers occurs, the demarcation between them 
disappears, and the upper layer, as Assauron (1909) has already 
observed, is incorporated in the wall of the brain and the medullary 
canal. In the praecerebral part of the apical plate however the 
coherence between the upper and lower layers becomes less and 
less, which no doubt is connected with the circumstance that this 
part of the ectoderm has to overgrow the cerebral plate. The lower 
layer finally lies as a compact cell-mass under the upper layer, 
which acts as ectoderm, and quite dissociated from it (fig. 4 
pr. cer.). Judging from Kuprrer’s (1906) figures of the later stages, 
it is this cell-mass which moving under the brain vesicle, ultimately 
gives rise to the hypophysis. A possible relation between the origin 
of the hypophysis and the animal pole in vertebrates would no 
doubt be worth closer examination. | 
If now we revert to the bottom of the body we see that here 
too the median sections of figs. 3 and 4 differ more from each other 
than paramedian ones do. The anus has broken through, the ventral 
blastopore lip accordingly seems to have vanished at once. The 
blastopore itself has been overgrown by the medullary folds. In the 
posterior part of the medullary tube the latter have applied them- 
selves so closely one to the other, that the lumen of the tube is not 
continued between them and only a virtual neurenteric canal can 
be spoken of. Later, judging from the diagrams of other investigators, a 
lumen seems to reappear and thus a real neurenteric canal. SIDEBOTHAM 
and ERrLANGER give diagrams of median sections of eggs in which 
the anus is just on the point of breaking through. From the study 
of whole eggs it appears quite evident that the medullary folds unite 
