MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 405 



In Fig. 98 several independent chromatiu balls (S) are seen in the yolk, and the granular 

 cells of I In- ventral plate are very marked. A large nucleus of one of the cells (ec.), which contains 

 sc\cral sporelike bodies, is very irregular in shape, owing to the pressure to which it is subjected, 

 and it lias evidently been crowded down from a level nearer the surface. The cell protoplasm, 

 however, shows that it still belongs to the surface tier of cells. This is also true of the small len- 

 ticular nucleus next it on its anterior side. Just anterior to this cell is another (Ec. 1 ) which is in 

 the act of dividing in the usual way. It seems probable that had this division been effected one 

 or more of the adjacent cell.s must have been crowded quite below the surface. It is difficult, 

 however, to always determine whether cells whose nuclei are a considerable, distance below the 

 surface do not SIMM! it]> stands of protoplasm to meet it. We find in the ventral plate, inde- 

 pendent cells multiplying by karyokinesis, and, as evidences of delamination in this region are rare 

 or altogether wanting, we are convinced that, as in the case of the optic disks, this thickening is 

 largely due to the migration of cells from the surface. 



There is at this stage a fairly well-defined sheet of cells (Figs. 99, 100, Mes.) extending forward 

 from the ventral plate on either side. The nuclei are oval or elongated, and their long axes are 

 parallel with the surface, that is, at right angles to the major axes of the superficial epiblast cells. 

 This layer of cells is most marked at the bases of the appendages (Fig. 100, Mes.) and extends 

 from the optic disk on either side to the ventral plate. The question of the origin of these cells 

 is not difficult. They are wandering cells which have settled down on these parts of the embryo. 

 They form a part of the future mesoblastic tissues ; exactly what part they play will be discussed 

 later on. They multiply by indirect division and extend into the folds of the appendages, while 

 some, on the other hand, degenerate. 



STAGE VI. THE EGG-NATJPLIUS. 



The fully developed egg-uauplius is represented in Fig. Ill, but before this condition is reached 

 there are several intermediate stages to be considered. A series of longitudinal sections (PI. 

 xxxix, Figs. 101-105) illustrates the structure of an embryo twelve and a half hours older than 

 the one last described. The thoracic-abdominal fold (Fig. 104, Ab.) can now be recognized, and 

 the stomodieiim (Fig. 105, Std.) has the form of a straight, narrow tube, between the buds of the 

 first and second pairs of antenna?. The space between these two structures is tilled with yolk 

 fragments, among which are scattered, numerous chrouiatiu particles (S) and cells derived from 

 the thoracic-abdominal fold. The epiblast of the sternal region (Fig. 105, 98, St. A.) is no longer 

 a simple layer, but is slightly thickened. This thickening seems to be partly due to rapid cell 

 division in one plane. The cell nuclei are elongated or wedge-shaped and stand perpendicular to 

 the surface. As will be seen later it is also due to the accession of wandering, mesoblastic cells. 



The stomodaniiu is a relatively long straight tube with very slight lumen, and is surrounded 

 with chromatiu grains and scattered cells. It is formed at a considerable distance in front of the 

 point of invagination, and one to two days earlier than the proctodajnm. 



The thoracic-abdominal fold seems to arise by the sinking down of the epiblast along a defi- 

 nite, line. There is thus formed a narrow transverse pocket (Fig. 10G, Ab. C.), which is quite deep 

 and perpendicular with the surface. Numerous cells continue to pass from the thoracic-abdom- 

 inal fold to various parts of the embryo, and to join the sheets of cells (Fig. 1 <>.">, Mes.) already 

 mentioned. In Fig. 103 the four segments of the embryo are well shown. Th'is section crosses 

 the, optic disc (O. G.), the buds of the three appendages, and the edge of the thoracic-abdominal 

 fold. The sheet of mesoblastic cells (Mes.) is most marked opposite the folds of the appendages. 



The optic disks are now large masses of cells united by a transverse cord which is thickened 

 slightly on the middle line. In Kig. Klli, three cells are met in karyokinesis, one in the abdominal 

 area and two side by side in the optic disks. The former exemplifies the common method of cell 

 division, while the latter is a good example of the less common delamination. As has been already 

 seen, karyokinetic figures of dividing cells are commonly met with in Alpheus at all stages, except- 

 ing the, species Alpheus minor, where the division is at first probably direct. I have seen unclear 

 figures at, the yolk segmentation stage of ('nnii/on, also in Hippa, Pontonia and Homarus, and Rei- 

 cheubach found them in abundance in Astacus. Indirect cell division is undoubtedly the rule in 



