408 MEMOIRS OF TOE NATIONAL ACADEMY OP SCIENCES. 



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

 rounded by a small irregular body of protoplasm which does not readily stain and which is often 

 difiScult to observe. In Pl.xxix, Fig. 33, several of these wandering cells are seeu greatlj enlarged. 

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

 cases there appears to be no such conuectiou between them. 



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

 layer at this time. In an embryo a few hours ohler 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 entoblast 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-uauplias (Fig. Ill) and the present 

 stage (Fig. 130), aud 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. 13G, 137, 144, 145. 

 Fig. 137, which represents a section just behind the base of the first antennai (A. 1.), may be com- 

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

 as will be seen (Fig. 134, Mu.), 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, aud 

 also by subsequent multiplication in tiie 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. 



Tlie 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 aud second pair of autenufe, aud mandibles are all simple appen- 

 dages, and are quite similar iu 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 

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

 drawn closer together aud arch outward from the middle line. The anus is doi'sal. Tiie abdomen 

 extends forward until it nearly meets the labrum 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, aud the second antenuse 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 ntimately united to the 

 brain. The anus is terminal. On at least the first pair of antenn;e hairs are developed, although 

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

 would appear from the figure. The first and secoud maxillfe and the first and second maxillipeds 

 are present aa rudimentary buds. 



Tlie 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 dowu to 

 I)articles no larger than the balls of chromatin which are suspended within the nucleus, and from 

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

 agree in containing coarse grains of chromatin or nucleoli. These vary much in size and number 

 in different nuclei ac(M)rding to the condition of the cell. In degenerating nuclei, the chromatin 

 residue is aggregated iuto fewer aud larger masses. 



