MEMOIRS OF TUB NATIONAL ACADEMY OP SCIENCES. 41 1 



circum-cesophageal commissures to the first post-oral ganglia. The fibrous masses unite on either 

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

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



All the segments of the body are now marked oft' as seen in Fig. l.'J9. The lirst pout-oral 

 segment is the mandibular (g. IV), and following it are the segments of the maxilhe the maxilli- 

 peds and the first pereiopod. The 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 thoracic-abdominal 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, which are abundant in all later stages. 



The optic lobe consists of two sharply distinguished parts already mentioned, the retinal and 

 jjanglioiiic portions. The retiuogeu which forms the 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 thinning out 

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

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

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

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

 est portion of the 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 intensely 

 (v. Section ix). 



The heart (Fig. 139, H) is now a broad and greatly flattened chamber between the body wall 

 and endoderm (End). It extends forward a considerable distance between the epiblast and yolk, 

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

 and blood corpuscles. 



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

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

 the eudoderin plate, and other parts of the embryo (Fig. 139, 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 PL 

 xxvii, Fig. 158. The optic lobes are huge pear-shaped masses meeting on the middle line in 

 front and arching outward and backward on either side of the brain. The ocular 1 pigment appears 

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

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

 convex end. The brain is constricted into two portions corresponding to the auteunular and 

 anteunal segments. 



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

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

 larva, PI. xxi, Fig. 9.) The plumose seta- which garnish the posterior edge of the telson are now 

 represented by short stumps. 



The first pair of antenna 1 are stout, simple appendages, tipped with seta 1 and folded backward 

 along the sides of the body. The second pair of antennie just inside of the latter, are biramous. 

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

 a glove. 



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

 embryos, all of which are of the same age, excepting those represented by Figs. 152, 15S, 15!>, and 

 101, which are a trifle more advanced. 



In the first series (PL XLVI, Figs. 146-151) the pigment cells are .just forming in the eye. They 

 are first developed in the thickest part of the retiuogeu next the food yolk. A single section, like 



