214 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



undivided. A single nucleus was visible in the small segment, but the egg in reality 

 contained six cells, five lying in the unsegmented yolk. 



It would be interesting to know how many of the irregularly segmented cells 

 eventually attain a normal condition. It seems probable that very many of them do, 

 judging from the fact that the number of abnormal eggs which later appear when the 

 nauplius stage is reached is much smaller, yet there is no evidence that any of the 

 eggs are lost. 



THE INVAGINATION AND EGG-NAUPLIUS STAGES. 



I will now speak of some interesting variations which occur during the invagina- 

 tion period and immediately after it. 



Instead of the normal ingrowth of cells from the surface into the yolk and the 

 sinking in of others to form a small circular pit, there is what appears at the surface as 

 a deep transverse invagination. This is sometimes a long crescent-shaped or irregular 

 transverse fissure, as in the egg of which cut 40 represents a median longitudinal 

 section. 



In other cases, in which the processes of development have gone further, there is 

 formed an irregular, oval, or circular disk of cells in connection with the invagination, 

 as shown in fig. 229, plate 51. Here there is a well-defined rim on one side, while upon 

 the other the structure seems to blend with the yolk. In a further-developed stage in 

 the same process I find that the egg has often a well-defined, sometimes round, and 

 very irregular circumvallate disk of cells. The cells within the vallum are densely 

 crowded, and the presence of numerous karyokinetic figures shows that at times cell 

 division may become rapid. Below the surface, both within and without the vallum, 

 the granular masses of chromatin bear abundant testimony to the degeneration of living 

 protoplasm which is taking place in the yolk. The columnar aspect of the marginal 

 cells of the disk can be plainly seen. The way in which this condition is reached is 

 illustrated by cuts 39 and 40. By the ingrowth (or infolding in consequence of unequal 

 growth) of some of the superficial or ectoblastic cells into the massive ball of yolk, a 

 tongue-shaped or island-like patch of cells is formed, on which the embryo proper is 

 subsequently marked off (figs. 228 to 231). 



The egg-nauplius may arise in a depressed central part of the disk, as in fig. 231, 

 or upon its margins, figs. 228, 230. 



We will now glance at the histology of some of the abnormal embryos. Out 40 

 shows a median longitudinal section through one of the earlier stages described. 

 When the egg was examined from the surface a transverse irregular fissure was seen, 

 corresponding to the pit {Pit) where the sheet of cells dips below the surface. We see 

 from a study of this egg that a considerable stratum of cells, including the invaginate 

 area, has grown into the yolk, and that its edges are folded upon themselves. In this 

 case one side of the disk, corresponding to the anterior end of the embryo, is at the 

 surface, while the opposite side is deeply embedded in the yolk. Numerous cells have 

 budded off from this cell plate, particularly at its posterior end, where they multiply 

 rapidly and move about freely in the yolk, like the normal mesendodermic cells. Like 

 the latter, they move chiefly in a posterior direction into the deeper parts of the yolk. 

 Many of these wandering cells are moreover already in process of degeneration. It 

 looks as if there was a migration of cells from the surface behind the cell plate, but 

 the appearances may be in this respect deceptive. The yolk flows over the engulfed 

 cells, but I find in my preparations no new superficial layer of ectoderm established. 



