414 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



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

 pit niarkiny the point of ingrowth. As the iuvagiuated cells pass into the yolk they degenerate, 

 giving rise to spore like particles which spread in incredible numbers tlirongh a large part of the 

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

 A part of a similar section is shown in more detail in PI. v, Fig. 30. The i)articles vary consider- 

 ably in size, stain uniformly and intensely and the yolk about them is granular or iinely divide<l. 

 At a corresponding stage in the lobster (Homaru.s americanun), I have observed a large diffuse 

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

 ou the side of the oblong egg and the cell plate is at one end of it, at a point about 90"^ behind tiie 

 embryo. This jiositiou 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 [)air of abdominal appendages are present in a rudimentary 

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

 optic lobes. The embryonic telson is fringed with seven pairs of setse, and resembles the larval 

 telson, except that the median notch is deeper. Seen from the exterior the eye-pigment has the 

 form of an oval disk. 



The longitudinal section, PI. XLVili, Fig. 16S, shows most of the important changes which have 

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



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

 cover its whole inner convex surface (PI. xlviii, Fig. 107). Near the outer surface of the eye the 

 crystalline cone mother cells («;) can be lecognized, and between the eye and the ganglia of the optic 

 lobe there is a narrow space which communicates freely with the blood sinus (/>'. S.) 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 find their way in thither from the yolk. In 

 the optic lobe another tibrous mass has ileveloped near the eye (Fig. l(>4-7). In horizontal section 

 (PI. XLVii, Fig. 1.59) the relations of the tibrous tissue of the brain and optic lobes is clearly shown. 

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

 or optic swelling of the brain (of). 



The structure of the brain (Figs. 159, 169, 170) begins to apjiroach in coujplexity that of the 

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

 cons|>icuous in the later stages have now appeared (Fig. 159 and PI. XLIX, Fig. 174, //'.). They are 

 developed in close union with the large central tibrous mass, which su))plies the optic lobes, and 

 probably belong primarily to the autennular segment. Beh)W this and nearer the middle line there 

 is a less definite fibrous center (g/".) which supplies the autennal segment. With this, the oeso- 

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



The complete chain of ventral ganglia can be seen in Fig. 16s. This section is not jjerfectly 

 median, but cuts a fiber-ball of each ganglion. The skin or hypodermis 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 jjeculiar cells, the nuclei of which aie spindle- 

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

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

 cord in the thoracic region is shown in Fig. 172, 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 tibrous 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 endodermal fold (/) near the point of union of the iliesenteron 

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

 served in much earlier stages. 



