364 WHEELER. (Vor. Ii. 
soon as the growth in this direction is fairly started the ecto- 
derm bulges out (Fig. 83 ect) and, drawing with it the somatic 
mesoderm, leaves a cavity between the yolk and the embryo 
which soon communicates with the coelomic cavity and assumes 
large dimensions (Fig. 84). Through the body cavity thus 
formed a thin plasma found as coagulated masses in hardened 
embryos probably circulates. 
The cell cd (Fig. 83), which is recognized even at a much 
earlier stage by its peculiar form, and which is destined to take 
part in the formation of the heart, is the only element still 
uniting the splanchnic and somatic layers. This cell, as may be 
clearly seen in Figs. 83 and 84 cé, is triangular in cross-section, 
and inserts one of its acute angles between the yolk and the 
ectoderm. As this cell with those of exactly the same shape 
anterior and posterior toit form the heart, I shall call it a cardio- 
blast. The true shape of the cardioblasts may be seen ina thick 
frontal section (Fig. 89) through the embryo of which Fig. 84 is 
a cross-section. Here the compact row of crowded but regular 
heart-forming elements (cd) is seen running between the actively 
proliferating entoderm cells (ez¢) on the one hand, and the meso- 
derm cells (#zsd@) on the other. These last are somewhat scat- 
tered in the space between the cardioblasts and the thickened 
ectoderm. 
In the more advanced embryo (Fig. 85) the cardioblasts (cd) 
from either side are near together. They have retained their 
characteristic form and position, while the somatic mesoderm 
has been converted into muscles (#s/) and connective tissue, 
and the splanchnic layer (s/vz) has applied itself closely to and 
is coextensive with the single-celled layer of entoderm (e772). 
In Fig. 86 the heart is completed. A glance at this figure and 
Fig. 85 shows the manner in which the two cells from opposite 
sides unite. Though forming the two halves of the tube in 
Fig. 86, they still show the three angles which were apparent 
just after the formation of the ccelomic cavity. The heart 
remains in the condition shown in Fig. 86 till the embryo 
hatches. I have not studied the formation of the blood in 
Doryphora. 
My observation on the formation of the sexual organs, though 
more complete than in 4/a/za, are still very fragmentary. These 
organs originate as two elongate thickenings of splanchnic 
