MEMOIKS OP THE NATIONAL ACADEMY OF SCIENCES. 405 



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

 cells ol" t 111' ventral jilate are very marked. A large nucleus of one of the cells (ec), which contains 

 several sporelike bodies, is very irregular in shape, owing to the pressure to which it is subjected, 

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

 however, sliows 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.') 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 cells must have been crowded quite below the surface. It is diflicult, 

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

 surface do not send up stands of i>rotoi)lasm to meet it. We find in the ventral i)Iate, 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 tliiekening is 

 largely due to the migration of (h^IIs from the surface. 



There is at this stage a fairly well-defiued sheet of cells (Figs. 9!l, 100, Mes.) extending forward 

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

 parallel with the surface, that is, at right a.ugles to the major axes of the superticial ei)iblast cells. 

 This layer of cells is most marked at the bases of the a]>pendages (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 (lillicult. 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 YI. — The eggnauplius. 



The fully developed eggnauplius is represented in Fig. Ill, but before this condition is reached 

 tj^ere are several intermediate stages to be considered. A series of longitudiiiiil sections (PI. 

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

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

 the stoiiioda'um (Fig. 10."), Sid.) has the form of a straight, narrow tube, between the buds of the 

 first and second jiairs of antenna'. The space between these two structures is filleil with yolk 

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

 the tlioraeie-alxloniinal fold. The epiblast of the sternal region (Fig. 10.5, 98, St. A.) is no hniger 

 a simjile 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 (-ells. 



The stoinoda'um is a relatively long straight tube with very slight lumen, and is surrounded 

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

 ])oiiit of invagination, and one to two days earlier than the i)roctoda'um. 



The thoracicabdomiiial fold seems to arise by the sinknig down of the epiblast along a defi- 

 nite line. There is thus formed a narrow transverse pocket (Fig. 106, Ab. C), which is quite deep 

 and perpendicular wiih the surface. Numerons cells continue to pass from the thoracic-abdom- 

 inal fold to various parts of the embryo, and to join the sheets of cells (Fig. 10,?, Mes.) already 

 mentioned. In l<'ig. 10;5 the four segments of the embryo are well shown. This section crosses 

 the optic disc ((). (i.), the buds of the three appendages, and the e<lge 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 Inie. In Fig. lOli, 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 Alpheux minor, where the division is at first probablj' direct. I have seen nuclear 

 figures at the yolk segmentation stat;e of Cranfion, also in ITippa, Poiifonia and Ilomnrus, ami Rei- 

 chenbach found tiiem in abundance in Astacuv. Indirect cell division is undoubtedly the rule iu 



