408 MEMOIRS OF TFTE NATIONAL ACADEMY OF SCIENCES. 



nuclei have the usual characteristics irregular shape and graunlar contents. They are sur- 

 ronmleil by a small irregular body of protoplasm which does uot readily stain and which is ofteu 

 difficult to observe. In PI. xxix, Fig. 33, several of these wandering cells are seen greatly enlarged. 

 Sometimes, as here, the perinuclear protoplasm appears to join that of neighboring cells, but in most 

 cases there appears to be no such connection between them. 



The eudoderin, though not represented in the drawings, makes its appearance as a distinct cell 

 layer at this time. In an embryo a few hours older than that represented in Fig. 125 the eudo- 

 derm has the form of a narrow sheet of rather large, cells, between the yolk and the rudimentary 

 heart, near the body wall. In the space corresponding to the heart, blood corpuscles can already 

 be detected, besides scattered mesoblastic cells. Both the latter and the eutoblast are derived 

 from the wandering cells which come out of the yolk. 



STAGE VII. RUDIMENTS OF SEVEN PAIRS OF APPENDAGES. 



Fig. 110 represents a phase intermediate between the egg-nauplins (Fig. Ill) and the present 

 stage (Fig. 130), and is of special interest for the light which it throws on the history of the wan- 

 dering cells. The structure of this embryo is illustrated by PI. XLIV, and Figs. 136, 137, 144, 145. 

 Fig. 137, which represents a section just behind the base of the first antenna; (A. I.), maybe com- 

 pared with Fig. 117. Numerous yolk elements are found in the vicinity of the u-sophagus, where, 

 as will be seen (Fig. 134, Mn.), they become speedily converted into muscle cells and somatic meso- 

 blast. In Fig. 136 several wandering cells attached to the body wall, have, all the characteristics 

 of blood corpuscles, a deep staining granular nucleus, and a clear irregular cell body. The blood 

 cell and muscle cell are both derived from wandering mesoblastic cells, and in the early stages of 

 their metamorphosis they resemble each other, so that it is not always possible to distinguish 

 them. Undoubted blood cells, however, have already made their appearance. 



Figs. 144, 145 show that the yolk' is pervaded by a great number of cells. These originated in 

 the way described, chiefly by migration from the ventral plate and thoracic-abdominal fold, and 

 also by subsequent multiplication in the yolk. At this period some of the cells migrate to the pole 

 of the egg opposite the embryo and apparently assist in forming a conspicuous dorsal plate (Dp.). 

 In this embryo they have not quite reached the surface. A structure is eventually formed which 

 reminds one of the "dorsal organs" of various Crustacea. This and the wandering cells will re- 

 ceive further consideration later on. 



The embryo of Stage VII is represented in Fig. 130, PI. XLIV. Figs. Ill, 109, 110, and 130 

 form a consecutive series of embryos, each but a few hours older than the preceding. In the first 

 (Fig. Ill) the optic lobes, first and second pair of an tenure, and mandibles are all simple appen- 

 dages, and are quite similar in general appearance. The abdomen and part of the thorax are 

 represented by a simple fold, the thoracic-abdominal fold. In the second embryo (Fig. 109) the 

 parts are more compact and the second pair of antenna; are forked at their tips. In the third 

 phase (Fig. 110) the optic lobes and abdomen exhibit the most rapid growth. The former are 

 drawn closer together and arch outward from the middle line. The anus is dorsal. The abdomen 

 extends forward until it nearly meets the labrnm and has a slight groove or depression at its ex- 

 tremity. All the appendages have assumed a more oblique position with respect to the long axis 

 of the body, and the second antennae are now the largest. 



In the fourth phase (Fig. 130) we see the same changes carried still further. The optic lobes 

 are large convex disks which join each other on the middle line and are utimately united to the 

 brain. The anus is terminal. On at least the first pair of antenna' hairs are developed, although 

 there is not perhaps so marked a contrast between the first and second antenna? in this respect as 

 would appear from the figure. The first and second maxillae and the first and second maxillipeds 

 are present as rudimentary buds. 



The general structure of the embryo (Figs. 129, 131) agrees with that of preceding stage. 

 The embryonic tissues consist of cells of various sizes and shapes, from quite large cells down to 

 particles no larger than the balls of chromatiu which are suspended within the nucleus, and from 

 the spherical to the lens-shaped, spindle-shaped, and wedge-shaped forms. Generally all the nuclei 

 agree in containing coarse grains of chromatiu or uucleoli. These vary much in size and number 

 in different nuclei according to the condition of the cell. In degenerating nuclei, the chromatiu 

 residue is aggregated into fewer and larger masses. 



