MKMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 401 



two patches ot ectohlast on citlier 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 oilier. (See Fig. 2.) 



The i»rincipal tell mass is the thoracic-abdominal i)Iate (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 <('I]s at that point. The optic disks are due to the rai)id multiplication of epi- 

 blastic cells around delinite centers. Each is Joined to the ventral plate by a lateral band or cord 

 of (sells (L. Cd.), on which the ajipendages are subsetjuently budded oft". A transverse cord (T. 

 ('d.) soon bridges over tlie space between the optic disks, thus inclosing a triangular area, whicth 

 t;orresi)onds largely to the sternal region of the adult. The extension of the invaginate cells below 

 the surface is only i)arlially indicated bj- the shaded nuclei. They advance forward and backward 

 from the jioint of ingrowth, but principally upward, that is, toward the center of the o.gg, along 

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

 one hemisi)here of the egg. It is V-sbaped, but the angle between the arms of the V varies nnich 

 in dittt'ient 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 aiii)reciably 

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

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

 cells. Their relations are well shown by sections through the entire egg (Figs. 5G, 51), 60), in which 

 we can still distinguish the jjrimary yolk cells (F. Y. C.) from the cells derived from the invagi- 

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

 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 irregidarly seg- 

 mented into balls (l''igs. (iO, (i3, 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 sonn^ of the cells of the ventral plate (PI. IX, Figs. 61, 63) tend to form a syn- 

 cytium, 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 colls 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 embiyo and entire egg, and Fig. 63 a part of a .section highly magnified through 

 the ventral jilate and region of ingrowth. The cells immediately below the surface (S. Y. (J.) 

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

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

 their neighborhood shows also that the cell ]irotoi)Iasm is rapidly absorbing food. A series of trans- 

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

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

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

 <i])tic disks (Figs. 64-(i7) (ronsist of a single layer of epiblast. Their cells are Hat and polygonal, 

 cell bcuindaries are distinct, and the buig 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 ritdimbnts of the appendages. 



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

 cords of cells (Ij. Cd.) uniting the ojitic, disks to the ventral ]>late the traces of two pairs of a])i>end- 

 ages can be made out — tlie fii'st pair of antenna- A (I.), and, elo.se to the ventral plate, the mandibles 

 (Md.). Some of the central cells (C. 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 o[itic ganglion. 



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

 S. Mis. 94 26 



