918 
Heer in the last edition of their “Lehrbuch” — during the con- 
traction undergoes a change in shape as indicated in fig. 10c and 
discussed already in my former paper. We see in this the backward 
growing out of the stomodaeum of annelids into the epichordal 
neural tube of chordates, projected as it were on a plane. In 
fie. 109 it has been indicated, how in craniotes, in addition to the 
epichordal neural plate, the praechordal cerebral plate is now added, 
while in acrania the condition of fig. 10f continues (Anat. Anz. T. 44.) 
How are now our statements for Rana esculenta to be brought 
into accordance with those for Rana fusca, how are they themselves 
to be interpreted and what are the points of difference from the latter 
species? Simply in this way, that 1 in Rana esculenta the egg contains 
more yolk or at least is less isolecithal in structure, and 2 that the 
wandering of the entoderm area, shown in fig. 10a and 6, here 
occurs later. 
Let us revert firstly once more to the annelids. In my article on 
the development of the annelid Scoloplos, published this year (1916), 
I have tried to show that among the eggs of polychaete annelids three 
types are to be distinguished. In the first place we have the small, 
poorly yolked eggs of Polygordius, Hydroides etc, in which the cleavage 
results in a very equal coeloblastula (Fig. 11a). Now in the larger 
eggs of other species two types of polarity may very early be 
recognized, which exert their influence on the here very determinate 
cleavage. In the first place the polar or radially symmetrical polarity, 
expressing itself in accumulation of yolk at the vegetative pole, which 
again causes the entoderm cells to be much larger than the cells of 
the three quartets of ectomeres. In the second place the bilateral 
a b c 
Fig. 11 a, b, c. Diagrammatic representation of the 3 types of polychaete eggs. J, 11, III = 1st, 2nd 
and 3d quartet of ectomeres, ent. = entomeres. 
a, minute, yolkless egg. b. egg with pronounced polar polarity. c. egg with pronounced bilateral polarity. 
