DEVELOPMENT OF SIMOCEPHALUS 639 
from the laterally lying mesoderm (fig. 11), at the anterior 
end no such clearness exists and at the blastozone the mesendo- 
derm merges into the plate of mesoderm lining the anterior 
part of the embryo. But both this anterior mesoderm and the 
mesendoderm clearly arise from a sharply defined group of 
cells at the blastozone, and it is suggested that there is no 
distinction between the mesoderm of the megendoderm and 
the other mesoderm formed in this anterior region. Tf this is 
80, a very complete analogy can be found with Polyphemus. 
Text-fig. 1 (b) shows a view of the vegetative pole of a P oly- 
phemus embryo in the thirty-two-cell stage. Two central 
primordial germ-cells forming the genital rudiment are placed 
posteriorly to two cells which give rise to the whole of the 
endoderm. laterally and posteriorly to the genital rudiment 
are six cells which give rise to both ectoderm and mesoderm. 
Kach of these six cells divides into two cells, one of which 
becomes an ectoderm cell and the other gives rise to mesoderm 
cells. In the comparison of these two figures it is seen in the 
two cases that the germ-cells are completely segregated in 
the genital rudiment as two cells in Polyphemus and as 
a group of about four cells in $8. vetulus. Forming a 
crescent posteriorly round this primordium in both cages are 
mesectodermal cells, but anteriorly in Polyphemus are 
two endoderm cells, while in $8. vetulus are a group of 
mesendoderm cells. The chief difference between the two 
forms is thus that the endoderm is segregated very late in 
5. vetulus, while it separates very early in Poly phemus— 
in the sixteen-cell stage. Similarly the mesoderm is segregated 
later than the endoderm, but still very early in Poly phemug 
compared with $. vetulus where the separation of mesoderm 
is only complete with the separation of the endoderm. 
In Moina and Polyphemus Weismann (15) has proved 
that the parents nourish the young in their brood-pouch, 
and it is probably due to this fact that the yolk in the eggs 
of these two forms has diminished so considerably, and in 
correlation with this disappearance of yolk is the appearance 
of the teloblastic type of development. In $8. vetulus 
Uae 
