402 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



several important changes since the last stage. The ventral plate is more extensive and the 

 wandering cells are more numerous. The primarj' and secondary wandering cells can no longer be 

 distinguished, but all the cells within the yolk are similar in character. They have large, granu- 

 lar nuclei, and multiply by ordinary karyokinesis (Fig. 75, Y. C^). Some pass forward underneath 

 the embryo (Fig. 74, Y. C.) and to other parts of the egg. In Fig." 71 two yolk elements are 

 found near the lateral cord, and in Fig. 70 one is seen just under the optic disk. lu Fig, 75 

 the section is ruedian and corresponds closely with the similar longitudinal section of the last 

 stage (Fig. 59.). The transverse cord (T. Cd.) is seen to consist of a single layer of flat epiblastic 

 cells. 



The structure of the ventral plate is shown in detail in Fig. 85, which is a section just in 

 front of the point of invagination. This illustrates the character of the syncytium beneath the 

 surface of the plate and the fine degree of fragmentation which the yolk suflers in the presence 

 of the cell protoplasm. At the surface, the cell walls are quite distinct, as already seen, but as 

 soon as the cells pass below it, the protoplasm extends itself in pseudopodialike processes and 

 incloses i)articles of yolk. Under these favorable conditions of nourishment these elements, which 

 nuist he regarded as the mother cells of the mesoderm and the endoderm, multiply rapidly and 

 spread to all i)arts of the egg. If this section is cotnpared with that of the invagiuate stage (Fig. 

 54), and with a similar section of Stage in (Fig. 61), it is easj' to understand the relations of the 

 ventral plate to the wandering cells, and the way in which the thickening of the plate is brought 

 about. At the second stage comparatively few cells take part in the invagination, and the most 

 of these pass directly into the yolk. But, almost simultaneously with this migration of cells there 

 occurs a migration of cells from the surface of the ventral plate. Thus this becomes thickened 

 and cells continue to be supplied to the yolk. This thickening of the plate is possibly due to cell 

 division in both planes ; that is, to delamiuation and emigration. (Comjiare cells EC, EC'^, Fig. 85.) 



The process by which the optic disk becomes thickeued at this stage is quite similar, although 

 there is no true invagination concerned in it. This is shown by a series of connective sections 

 (Figs. 76-83) passing through the entire disk. The anterior rim of the disk is cut in Fig. 76, and in 

 Fig. 83 the rudiment of the first antenna. This thickening is mainly the result of the migration of 

 epiblastic cells from the surface. After leaving the surface, the cell wall usually becomes indefi- 

 nite. The relations of the optic disks to the entire egg are seen in Fig. 84. The central yolk is 

 segmented as shown in Fig. 74, and a yolk cell usually lies within the segment. 



Stage V. — Rudiments of three pairs of appendages — cell degeneration. 



The embryo represented in Fig. 93 is, approximately, three days old (temperature at Nassau 

 78-80° F.). It occupies nearly one entire hemisphere of the egg, the opposite side of which is 

 covered with flat epithelial cells like those seen at the periphery of the figure. The shape of the 

 embryo proper is uearly that of an equilateral triangle, one angle of which corresponds to the 

 ventral plate, and the other two to the optic disks. A line drawn through the first of these angles 

 and the middle of the o])posite side would therefore correspond with the longitudinal median 

 axis of the embryo. 



The rudiments of the second pair of antennje, A (II), have now appeared, and we therefore 

 have present at this stage buds of three pairs of appendages, namely, the first and second pairs of 

 autenuai and the mandibles. All are developed nearly simultaneously, but the second pair of 

 antenniB seem to lag a little behind the rest. lu respect to the order of appearance of these 

 appendages, allied species of Crustacea differ slightly. The central parts of the ventral plate 

 ( Ab. P.) and optic disks (C. M.) are areas of rapid cell division, and are characterized by the presence 

 of large granular nuclei and by the irregular arrangement of the cells. In all other parts of the egg 

 the superficial cells form a uniform stratum one cell deep. This irregularity is due to the gradual 

 migration from the surface of individual cells in these three places. The first pair of antenute 

 are closely associated with the optic disks, and the mandibles abut against the ventral plate on 

 either side of the middle line. The appendages start on the lateral cords from definite centers of 

 cell division, and the cells tend to assume a radial and concentric arrangement around each center. 



The space between the optic disks is now completely bridged over by a sheet of closely 



