MEiMOlUS OF TUB NATIONAL ACADEMY OF SCIENCES. 411 



circam-OESophageal commissures to the first post-oral ganglia. The fibrous masses unite on either 

 side of the ventral ciiaiii to form a double longitudinal commissure. Thus at an early stage the 

 optic lobes, brain, and ventral nervous system are intimately connected by fibrous tissue. 



All the segments of the boily are now marked off as seen in Fig. 130. Tiie first jiost-oral 

 segment is tiie maiidil)ular (g. iv), and following it are the segments of the maxillic the maxiili- 

 peds and tiie first pereiopod. Tho second thoracic ganglion lies in the angle made by the 

 thoracic-abdominal flexure. The third, fourth, and fifth thoracic, and the six abdominal segments 

 constitute the thonicic-abdomiual fold. 



In the (superficial parts of the brain large elements are met with which multiply by mitosis, 

 but have a distinct histological character. They consist of a large spherical nucleus containing a 

 diffuse chromatin network, and a granular cell body without definite wall. They are the ganglion 

 cells, wiiicli are abundant in all later stages. 



The optic lobe consists of two sliaii)ly distinguished parts already mentioned, the retinni and 

 gangUnnic portions. The retinogen which forms tiie eye is a superficial disk of cells resting like a 

 cap on the other part of the lobe, thickest on the outer side of the lobe and rapidly iliinning out 

 toward the middle line to a single layer of cells. The nuclei are elongated and wedge siiaped, and 

 cell division takes place commonly in a plane perpendicular to the surface, corresponding to the 

 long axis of the vertical nucleus. The gangliogen consists of a deeper portion next the brain, 

 containing a ball of fibrous tissue, and a part next the retinogen or eye rudiment. Below the thiirk- 

 est portion of tlie eye the cells have large nuclei, which show a tendency to arrange themselves in 

 rows radiating from the deeper half of the lobe. These large, clearer cells also extend down to 

 the food yolk, and in lateral longitudinal .sections (Fig. 138) form the inner stratum of the lobe. 

 The cells which lie between them and the eye, here one cell thick, are smaller and stain inlen.sely 

 (v. Section ix). 



The heart (Fig. l.'i!), 77) is now a broad and greatly flatten* d chamber between the body wall 

 and endoderm {End). It extends forward a considerable distance between the ejiiblast and yolk, 

 and is continued backward into the superior abdominal artery {A. a. «.). It is tilled with serum 

 and lilood corpuscles. 



The endoderm is a more conspicuous layer {End.), and the wandering cells are reduced in nnin- 

 bers. They are still seen in all parts of the egg, approaching the body wall, the nervous system, 

 the endoderm plate, and other parts of the embryo (Fig. 131), y. c). 



Stage IX. — Embryo with eye pigment forming. 



A sketch of the embryo at the time when eye pigment has already formed is shown in IM. 

 XLVii, Fig. ir)S, The optic lobes are hnge pear-shaped masses meeting on the middle line iu 

 front and arcliing outward and backward ou either side of the brain. The ocular pigment appears 

 as a thin, dark-brown crescent near the outer surface of the lobe. Pigment is first formed at tho 

 posterior end of the lobe nearest the base of the antenuules, and spreads upward over its larger 

 convex eiul. The brain is constricted into two portions corresi)onding to the antennnlar and 

 antennal segments. 



The segments of the abdomen are faintly marked ofif at the surface, and the telson plate which 

 overlaps the month, is deeply forked at its extremity. (Compare with spatnlate telson of the first 

 larva, PI, xxi, Fig. 9.) The plumose set;E which garnish the posterior edge of the telson are now 

 represented by short stumps. 



The first pair of anteniKc are stout, simple appendages, tipped with setsp and folded backward 

 along the sides of the body. The second pair of anteiinie Just inside of the latter, are biramons. 

 They are also hairy at the tips, and the embryonic nieinbranes surround them like the fingers of 

 a glove. 



The present stage is illustrated by Pis. XLVI and xlvii. Tlie drawings are made from different 

 embryos, all of which are of the same age, excepting those represented by Figs. 152, 15s, 1.30, and 

 l(il, which are a trifle more advanced. 



In the first series(lM. xi.vi. Figs. HG-l.Tl) the pigment cells are just forming in the eye. They 

 are first developed iu the thickest part dl the retinogen next the food yolk. A single section, like 



