MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 401 



two patches of ectoblast on either side of the middle line in front of the ventral plate. When 

 the living egg is examined by reflected light these structures appear as misty white patches sep- 

 arate from each other. (See Fig. 2.) 



The principal cell mass is the thoracic-abdominal plate (Fig. 58, Ab. P.), from which the 

 thoracic-abdominal process is developed. The position of the pit (Ig.) is faintly marked by the 

 arrangement of the cells at that point. The optic disks are due to the rapid multiplication of epi- 

 blastic cells around definite .centers. Kadi is joined to the ventral plate, by a lateral band or cord 

 of cells (L. Cd.), on which the appendages are subsequently budded off. A transverse cord (T. 

 Cd.) soon bridges over the space between the optic disks, thus inclosing a triangular area, which 

 corresponds largely to the sternal region of the adult. The extension of the, in vagi n ate cells below 

 the surface is only partially indicated by the shaded nuclei. They advance forward and backward 

 from the point of ingrowth, but principally upward, that is, toward the center of the egg, along 

 the lines joining the optic disks to the ventral plate (Figs. 59, CO). The embryo covers nearly 

 one hemisphere of the egg. It is V shaped, but the angle between the arms of the V varies much 

 in different eggs. There is a marked contraction of the embryo which takes place immediately 

 after this phase, that is, the area of the surface occupied by the embryo becomes appreciably 

 smaller. A similar contraction of the embryo has been observed in Astacus (54) and Crangon (31). 

 With the extension of the epidermis there has been a corresponding activity among the wandering 

 cells. Their relations are well shown by sections through the entire egg (Figs. 56, 59, 60), in which 

 we can still distinguish the primary yolk cells (P. Y. C.) from the cells derived from the imagi- 

 nation (S. Y. C.). The cells of the first have large, granular nuclei and send out processes iuto 

 the yolk. The others are smaller and are probably multiplying more rapidly. It soon becomes 

 impossible to find any distinction between these wandering cells. The yolk is irregularly seg- 

 mented into balls (Figs. 60, 03, Y. B.), inside of which the migrating cells are usually found. The 

 epiblastic cells of the surface, which are the direct descendants of the yolk pyramids, have definite 

 boundaries, but some of the cells of the ventral plate (PI. ix, Figs. 61, 63) tend to form a syn- 

 cvtium, as already seen, while the wandering cells are independent, free-moving elements. 



The lateral section (Fig. 56) passes through the outer edge of the ventral plate, and the 

 next toward the middle line (Fig. 60) encounters a sheet of wandering cells. We see at a glance 

 that the migrating cells pervade the greater part of the egg, and that they pass out in all direc- 

 tions from the region of the ventral plate (Ab. P.). Fig. 59 represents a median longitudinal 

 section through the embryo and eiit ire egg, and Fig. 63 a part of a section highly magnified through 

 the \entral plate and region of ingrowth. The cells immediately below the surface (S. Y. C.) 

 are characterized by large and very granular nuclei, which stain with much less intensity than the 

 superficial cpiblast. This shows that they are multiplying rapidly, and the finely divided yolk in 

 their neighborhood shows also that the cell protoplasm is rapidly absorbing food. A series of trans- 

 verse sections of this embryo is given in PI. xxxin. The plane of section in Fig. 61 is oblique and 

 passes in a posterior direction. In Fig. 62 the lateral cords (L. Cd.) are crossed and numerous 

 wandering cells arc encountered, while anterior to this (Figs. 68, 69) the optic disks are cut. The 

 optic disks (Figs. 64-07) consist of a single layer of epiblast. Their cells are flat and polygonal, 

 cell boundaries are distinct, and the long axis of the oval nucleus is parallel with the surface. 

 When the cell divides this position is reversed, the plane of division being perpendicular to a 

 tangent at the surface. From the optic disk the eye and its ganglia are developed. 



STAGE IV. THICKENING OF THE OPTIC DISKS AND RUDIMENTS OF THE APPENDAGES. 



An embryo a few hours older than the last described is shown in Fig. 72. On the thickened 

 cords of cells (L. Cd.) unit ing the optic, disks to the ventral plate the traces of two pairs of append- 

 ages can be made out the first pair of antenna-, A (I.), and, close to the ventral plate, the mandibles 

 (Md.). Some of the central cells (('. M.) of the optic disk have large, granular nuclei. These 

 mark the area of most active cell division, and form an ingrowth or thickening, which is the rudi- 

 ment of the optic ganglion. 



Glancing at a scries of longitudinal sections through this egg (Figs. 70-71, 73-75), we notice 



S. Mis. 94 26 



