THE AMERICAN LOBSTER. 209 



through tlic 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 

 Reichenbach (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 inesendoderm 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. Ou 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 antenna?, 

 (2) mandibles, (3) second antennas, (4) first maxilla?, 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 flageilum of this appendage. The first antenna? remain single until just 

 before the time of hatching, when the inner branch or flageilum begins to grow out 

 from the inner lower surface of the primary stalk. The optic disks are flat areas of 

 rapid cell division. 



F. C. B. 1895—14 



