SUDLER : DEVELOPMENT OF PENILIA. 
127 
embryo but disappears before the animal is born leaving no trace 
of its existence in the adult. In a Penilia embryo at a stage rep¬ 
resented by Fig. 28 it shows its maximum development which is 
quite weak in comparison with other members of the Cladocera. At 
the stage referred to, it is composed of from fifteen to twenty cells, 
each of which is longer than broad and larger at the distal end 
than at the surface end. The whole organ is shaped somewhat 
like a truncated cone with a small depression at the smallest end 
where it comes to the surface. (Fig. 33.) It is situated in the 
middle line at the anterior dorsal angle of the embryo. Its posi¬ 
tion in Penilia is more anterior than in Moina. It appears about 
the same time in embryos of both animals but disappears much 
sooner in Penilia than in Moina, in which it is much stronger 
developed than in the former. 
The Ovary. 
The ovary of Penilia is of mesodermal origin. I have failed 
completely to find a genital cell from which the future genital 
organs originate. I have been unable also to distinguish any 
particular cell corresponding to the “ Grobben’sche zelle ” of 
Samassa. Lebedinsky and Samassa both agree with me in being 
unable to find any trace of a differentiation of any kind at such an 
early stage to form the genital organs. In Penilia certain meso¬ 
dermal cells lying to either side of the intestine become larger and 
their cytoplasm stains clearer than those surrounding them. These 
form a row in a position occupied by the ovary in the adult. 
Samassa finds the ovaries of Moina derived from four mesodermal 
cells, two on either side. These cells multiply and grow backward 
to form the ovary. In Penilia the origin of the ovary corresponds 
more closely to the origin of the same organ in Branchipus. 
According to Claus it originates here as paired rudiments lying 
in rows on either side of the digestive tract. 
The Heart. 
The heart of the adult Penilia is an oval-shaped body composed 
of about thirty semilunar shaped cells. It possesses a single pair 
of ostia situated toward the posterior end. The heart arises in the 
