414 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



is not yet formed. The plate is slightly thickeued at its center, where there is an inconspicuous 

 pit marking the point of ingrowth. As the invaginated cells pass into the yoik they degenerate, 

 giving rise to spore like particles which spread iu incredible numbers through a large part of the 

 egg. Some of the wandering cells in this region doubtless degenerate before reaching ihe surface. 

 A part of a similar section is shown in more detail in I'l. v, Fig. 30. The particles vary consider- 

 ably in si/e, stain uniformly and intensely and the yolk about them is granular or finely divided. 

 At a corresponding stage iu the lobster (Homarus americanus), I have observed a large difiiiM- 

 patch of cells which probably answers to the structure just described. In this case the embryo rests 

 on the side of the oblong egg and the cell plate is at oiie end of it, at a point about 90 behind the 

 embryo. This position seems to be quite constant, while in Alpheus the plate is nearly opposite 

 the embryo, at the stage when it is most conspicuous. 



STAGE X. EMBRYO WITH EYE-PIGMENT STRONGLY DEVELOPED AND THE POSTERIOR LOBES 



OF THE GASTRIC GLAND FORMING. 



All the thoracic limbs and the sixth pair of abdominal appendages are present in a rudimentary 

 condition. The abdomen has grown forward until the tip of the telsou now extends beyond Ihe 

 optic lobes. The embryonic telsou is fringed with seven pairs of seta-, and resembles the larval 

 tclson, except that the median notch is deeper. Seen from the exterior the eye-pigment lias the 

 form of an oval disk. 



The longitudinal section, PI. XLVIII, Fig. 168, shows most of the important changes \\ liidi have 

 occurred since the last stage. These chiefly concern the eye, the nervous system, and the midgut. 



The eutodermal pigment cells (retinular cells) of the retinogen have spread inward until they 

 cover its whole inner convex surface (PI. XLVIII, Fig. 167). Near the outer surface of the eye the 

 crystalline cone mother cells (cc) can be recognized, and between the eye and the ganglia of the optic 

 lobe there is a narrow space which communicates freely with the blood sinus ( /,'. iS'.) on the outer side 

 of the lobe. Wandering cells are frequently seen rear this blood sinus, and in the sp.ice between 

 the eye and ganglia flattened cells also occur, which tind their way in thither from the yolk. In 

 the optic lobe another fibrous mass has developed near the eye (Fig. 104-7). In hori/ontal section 

 (PI. XLVII, Fig. 159) the relations of the fibrous tissue of the brain and optic lobes is clearly .shown. 

 In each lobe there is a chain of four fibrous masses united by a stalk of fibers to the anterior 

 or optic swelling of the brain (of). 



The structure of the brain (Figs. 159, 109, 170) begins to approach iu complexity that of the 

 larva, which was described in the first section of Part n of this paper. The lateral fiber-balls, so 

 conspicuous in the later stages have now appeared (Fig. 159 and LM. xnx, Fig. 174, //'.). They are 

 developed in close union with the large central fibrous mass, which supplies the optic lobes, and 

 probably belong primarily to the antenuular segment. Below this and nearer the middle line there 

 is a less definite fibrous center (///'.) which supplies the antennal segment. With this, the uiso- 

 phageal commissures are directly continuous (Figs. 171, 174 ocm.). 



The complete chain of ventral ganglia can.be seen in Fig. 10S. This section is not perfectly 

 median, but cuts a fiber-ball of each ganglion. The skin or hypoderuiis is now differentiated 

 from the nerve elements and consists of a thin layer of flat cells. The fibrous masses of the gan 

 glionic chain are also imperfectly surrounded by peculiar cells, the nuclei of which are spindle 

 shaped in section. These also occur iu the brain, and in either case must be regarded as metamor- 

 phosed ectoderm cells, or more probably as intrusive uiesoblast. A transverse section of the nerve 

 cord in the thoracic region is shown in Fig. 1713, and corresponds very nearly in plane of section 

 with Fig. 151 of the last stage. The fusion of the ganglia is now more complete, and the fibrous 

 balls and commissure are relatively larger. (Compare with this, Fig. 176, a section of the thoracic 

 ganglion of the larva.) In the abdominal ganglia the fibrous elements have essentially the same 

 relations, but lie at a deeper level, being separated from the adjoining tissues by at most a single 

 layer of nerve cells. In Fig. 168 we see that the yolk comes in close relations with the nerve 

 cord behind the oesophagus to the eudodermal fold (/) near the point of union of the mesenterou 

 and hindgut. Wandering cells approach the cord and become flattened against it, as already ob- 

 served iu much earlier stages. 



