MEMOIRS OF THE NATIONAL AOADExMY OF SCIENCES. 413 



cells. This subject of the r61e of the wandering celliiiu Alpheus is one of the most difficult and 

 at the same time the most interesting which has been met with m the study of its life history, 

 and a full discnssion of it is reserved for another part of this jiajier (Section Vil). 



A new structure, the carapace (I'Mgs. I4.S-ir)l, IW), c/*.), is seen for the tirst time at this period. 

 It takes the form of a lateral longitudinal outgrowth of ectoderm on either side of the body in the 

 region of the thorax. These longitudinal folds represent the branohiostegites, which form tiie outer 

 wall of the branchial cavity. In Fig. IGO the structure of the rudimentary carajiace and the way in 

 which it originates is very clearlj' shown. The epiblast cells at the surface multiply and the cell 

 juotoplasm is prolonged downward into long strands or spindles. Meantime the ectobiast is 

 pushed outward along definite lines and the spindles of one side of this fold unite with those of 

 the side opposite, thus forming a framework of transverse beams or pillars. Blood enters the 

 fold, which thus becomes a respiratory organ. This structure is essentially maintained up to lar- 

 val life (V. Fig. 195). 



The hindgut or intestine (Figs. 150, 157, Eg.) has a considerably larger lumen than in the i)re- 

 ceding stage, but its histology is essentially the same. The walls are composed of a single tier of 

 large columnar cells. The cell protoplasm is granular, and projects into the lumen of the tube, 

 and the cell wall is usually distinct. There is an outer investment of mesoblast as in earlier 

 stages, which is closely associated with tlie surrouuiling cells of the developing abdominal muscles 

 (Figs. 150, ICO, mu.f.). The muscle tibre appears as an outgrowth of the cell protoplasm of the 

 muscle cell. The nucleus elongates until it becomes rod-shaped. The fiber is homogeneous, 

 nonstriated and stains feebly in borax carmine, but not at all in hsemotoxylon. The intestine com- 

 municates with the midgut or the cavity which contains the great mass of food jolk, and with 

 the exterior by the anus, which is on the under side of the tclson plate opposite the labrum. 



Fig. 157 shows the continuity of the epithelia of the hind and midguts, and illustrates in a very 

 satisfactory manner the origin of the endoderm. Histologically the entoblastic cells resemble 

 those of the adjoining epiblast, and it is impossible to draw a sharp line between them. The 

 endo<lerm cells are vesiculated, have less definite boundaries, and extend pseudopodialike pro- 

 ces.ses into the yolk. The endoderm begins near the point marked tuic. Fig. 157. Here the lumen 

 of the tube eidarges, and the endoderm extends forward over the flexure of the nerve cord and 

 upward over the sides of the body. Posteriorly it is separated from the body wall by the blood 

 sinus which represents the heart and its arteries (H). The yolk next the endoderm is eroded and 

 granulated. The formation of the endoderm thus begins near the abdominal flexure, in the egg- 

 naujilius stage, at the point where the hindgut communicates with the cavity of the mesenteron, 

 and advan(;es gradually forward on all sides. It is comjiosed of cells (Fig.157, y. <:) which migrate 

 from the yolk and assume an extreme peripheral position with respect to it. They eventually 

 acquire cell walls, unite and inclose the yolk which they continue to feed upon, ajiparently by first 

 producing chemical changes in it and then absorbing its particles. In the early stages the 

 mother cells of the endoderm and mesoderm can not be distinguished w ith certainty. However, 

 since the endoderm originates as a distinct epithelial layer behind the ventral plate, and thence 

 spreads forwards, slowly walling in the mesenteron, it is obvious that the bulk of the anteriorly 

 wandering cells are mesoblast. 



The extensive blood sinuses which are now present have already been mentioned. In the 

 angle between the optic lobes in front, there is a large blood space (Fig. 15-', b. s.), and blood 

 passes from the heart upward and forward nearly around the egg in a thin irregular stream 

 between the skin and food yolk. Mesoblast cells grow forward, from the abdominal region and 

 line the outer sides of the endodermal wall, and exteiul u])ward to a thin .sheet between the yolk 

 and the epiblast. Endodermal cells coming from the jolk attach themselves to this layer. The 

 pulsatile chamber of the heart is not a very definite structure at this stage. It lies above the 

 endoderm, nearly opposite the angle of the thoracic-abdominal fold. Its walls are delicate, and 

 appear in sections as thin strands of mesoderm cells. The nuclei are elongated and the cell proto- 

 plasm is produced into long processes. The pericardial space surrounding it is filled with blood. 



In Stage Vil (Figs. 144, 145, Dp.) we noticed an extraordinary migration of waiidtTiiig cells 

 to the pole of the egg opposite the embryo. Tlie.se cells eventually reach the surface and rein- 

 forcing the [Iiimitive ejjiblast, givt; rise to a conspicuous dorsal i)I.Tte, which is shown in l"ig. 153 

 (Dp.). This is from an embryo intermediate between Stages viii and ix, in which eye-pigment 



