128 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
mesoderm in a position in the embryo corresponding to that occupied 
by it in the adult. Two lots of cells, one on either side of the 
middle line and above the intestine, become longer and tend to meet 
in a mid-dorsal position. These gradually become longer and more 
curved. These ends meet in the middle line and the structure of 
the heart is practically complete as seen in the adult (Figs. 1 and 
34 ). 
Conclusion. 
In reviewing the general features of the embryology of Penilia 
certain characteristics present themselves prominently throughout 
its development. The orientation of the egg is one of these. 
An embryo always arises whose anterior end corresponds to the 
same end of the egg and whose long axis also agrees with that of 
the egg. Whether this is brought about by inherent differentiation 
of the egg protoplasm or whether by external influences is impos¬ 
sible of demonstration. The egg of Penilia divides transversely to 
the long axis of the egg into two cells, and judging from the position 
of these two cells, the descendants of the anterior one must have 
most to do with the formation of the anterior end of the embryo and 
the posterior one most to do with the formation of the posterior end 
of the embryo. The first two cells are certainly not right and left 
in that their descendants form those halves of the body of the adult, 
as Roux found for the frog’s egg or Watase for cephalopod eggs, 
but rather as Wilson found for Nereis, where the first cleavage is 
also transverse to the future animal. 
Grobben oriented the egg of Moina by means of the polar body 
which lies in the substance of the egg. This body marks the ani¬ 
mal pole, and the cell containing it in the two-celled embryo by its 
development gives rise to the left side of the animal. 
The segmentation of Penilia is total and remains so throughout, 
in marked contrast to most of the Crustacea. Lucifer and Euphau- 
sia are the known exceptions in the higher forms. 
Much less yolk is present in the egg of Penilia than in most 
Crustacea even in nearly related forms, and the segmentation cavity 
never at any time possesses a yolk plug as is found in Moina. 
One of the facts most frequently quoted from Grobben’s article is 
the very early differentiation of one cell destined to give rise to the 
reproductive organs. Samassa also finds in Moina the same cell, 
