THE AMERICAN LOBSTER. 
209 
through the longitudinal median axis. There is no tendency to form radial strings 
or concentric circles of cells with reference to a given center at the surface, such as 
Eeichenbach ( 163 ) has described in the crayfish, a fact already noticed by Bumpus ( 30 ). 
Diffused clouds or islands of chromatin particles, the wrecks of cell colonies, lie strewn 
over the embryonic area, particularly in its forward part. These are for the most part 
immediately below the ectoderm. The opposite side of this egg shows nothing par- 
ticularly noteworthy and has not been figured. The nuclei are more scattered, cell 
division is less frequent, and clouds of chromatin granules are much less extensive. 
The internal structure of a little older embryo is illustrated by fig. 255, plate 54. 
The most noticeable changes are the great spread of the mesendoderm which, like a 
cloud of dense smoke from an engine, rises up and trails backward into the depths of 
the yolk with many rounded summits; the columnar form of the ectodermic cells — most 
pronounced in the region of the optic disks — and the swarm of degenerating par- 
ticles which underlie these regions. Sticking to the basal ends of the prismatic cells, 
numerous amoeboid elements can also be seen. How do they originate? They must 
come either from the mesendoderm or from the ectoderm. That some of them migrate 
forward from the region of the thoracic-abdominal plate there can be no doubt, and it 
seems almost equally certain that some come from the surface cells. The position of 
the nuclei of the peripheral cells frequently points to the theory that some of them are 
crowded below the surface by mutual pressure. On the other hand it is sometimes, but 
not always, the case that the boundaries of the ectodermic cells are clearly defined. 
The ectoderm still consists of a single cell stratum. The ectodermic nucleus is sus- 
pended in the middle of the cell, cytoplasm filling the peripheral and deutoplasm 
the central ends. Mesendoderm cells also travel backward and sideways from the 
thoracic-abdominal plate and settle down upon the ectoderm. The cells which migrate 
into the depths of the egg and form the cumulus-like mass have this peculiarity — 
they form a connected syncytial mass; their nuclei are small and of irregular shapes. 
On the other hand amoeboid cells below the embryonic area frequently possess large 
spherical nuclei. 
LATER STAGES IN EMBRYONIC DEVELOPMENT. 
The development of the external form of the embryo is illustrated by cuts 27 to 
38 and by plate 51. The mesendoderm cells play an important role at the time the 
appendages are budding. In surface views they become less and less conspicuous, 
until in the late egg nauplius (cuts 31, 32) they have passed out of sight into the 
deeper parts of the egg. 
The appendages make their appearance in the following order : (1) First antennae, 
(2) mandibles, (3) second antenme, (4) first maxilke, and the remaining thoracic 
appendages in regular succession. They are all formed by the folding of the body 
wall or ectoderm, and contain solid yolk cores, until these are absorbed and replaced in 
part by the mesodermic cells which migrate into them. The second antenna soon 
becomes bifid and bilobed at its apex (cuts 30-32), the inner branch representing the 
future long flagellum of this appendage. The first antennae remain single until just 
before the time of hatching, when the inner branch or flagellum begins to grow out 
from the inner lower surface of the primary stalk. The optic disks are flat areas of 
rapid cell division. 
I'. C. B. 1895-14 
