KO. 1783. NORTH AMERICAN LERN/EOPODIDM— WILSON. 199 



is now very difficult to trace any of the migrating material within the 

 yolk. The disk remains thicker over the point where the cells origi- 

 nally appeared, but elsewhere is only a single cell in thickness. When 

 it is finally completed (fig. 8) its cells are very much reduced in size, 

 and no accessory material can be seen around them; it has all entered 

 into the structure of the blastoderm. As soon as this has been accom- 

 plished the cells composing the blastoderm secrete a cuticular mem- 

 brane in addition to the vitelline and shell membranes. This has 

 been designated as the blastodermic cuticle, and according to Korsch- 

 elt and Heider its appearance can only be explained by regarding it 

 as a sort of ecdysis or molt, carried back to an early embryonic 

 period. The formation of this cuticle thus constitutes the first molt 

 of the Achtheres embryo. Such a cuticle is common among the 

 Malacostraca, but in the copepods is probably confined to those cases 

 like the present, in which many of the stages of development are 

 shifted back into embryonic life. 



FORMATION OP THE EMBRYO. 



Not until the blastoderm has entirely surrounded the yolk is there 

 any differentiation in it to indicate the position of the larval append- 

 ages. The first change is a considerable thickening of the cellular 

 layer which is to become the ventral surface of the embryo (the 

 "Keimstreif"). 



The multiplication of cells is especially rapid along the axes of the 

 future appendages and builds up there a series of lobes or pads which 

 constitute the "Kopfplatten" of the German embryologists. Ordi- 

 narily there are three pairs which have come to be known as the 

 nauplius appendages, which appear simultaneously and develop into 

 the locomotor organs of the first larval stage. These three develop 

 together into certain typical forms without reference to what may be 

 the shape and function of the matured organ which they represent. 

 For instance, the third pair are usually identical, whether they are to 

 become the gnawing mandibles of the free-swimmers or the piercing 

 mandibles of the parasites. Moreover, the three pairs develop and 

 have served their temporary function before the other appendages 

 appear. 



In the formation of the nauplius of the present species we notice 

 several radical departures from this established type. 



When a longitudinal section of the developing embryo is examined 

 it reveals nuclear centers not only for the first three appendages but 

 also for the other mouth-parts and the first two pairs of swimming legs 

 (fig. 9). These centers all appear simultaneously, but those of the 

 first two pairs develop much faster than the rest. And they form 

 typical nauplius appendages before any of the others have become 

 externally visible. The third pair never become nauplius appendages 



