OF OECANTHUS AND TELEAS. 237 



cylindrical cells, each of which appears polygonal in surface view and has near its centre 

 an oval coarsely granular nucleus. This plate of cells is about .7 mm. in length (pi. 18, 

 figs. 1, 2.) and protrudes beyond the surface of the surrounding blastoderm. The ventral 

 surface of this germinal baud is soon covered by a thin cell layer (amnion auct.). This 

 layer makes its appearance at the edges of the band soon after it is formed, and gradually 

 extends to the median line where its free margins coalesce. On account of the quantity of 

 the yolk and the size of its masses, the details of its growth vary from the usual manner of 

 formation. This variation is caused, as has been indicated, by the excessive amount of 

 food-yolk, which takes the form of large enuclear masses which become reduced in size only 

 by gradual assimilation within the amoeboid cells. While the germinal band is still a 

 single layer of cells the rest of the blastoderm, at its line of union with the band, pushes 

 up from its edge in the form of a fold ; but as the space between the germinal band and 

 the vitelline membrane is too narrow to allow the ingrowth of this fold in its primitive 

 condition, the lower or amniotic layer is retarded in its growth toward the middle line 

 and only assumes the nature of an embryonic membrane some time after the edges of the 

 upper layer (or serosa) have united, and all traces of its manner of formation have disap- 

 peared (pi. 23, fig. 8). The cells which form the amnion are given off from the lateral 

 edges of the germinal band, and even after the fusion of the free margins in the median line 

 this membrane lies closely pressed upon its surface until the appendages by their outward 

 growth push it off. The cells of the amnion finally assume the same polygonal form as the 

 serosa cells, (pi. 18, figs. 4 and 10,) but remain throughout their existence much smaller than 

 the latter. In sections of the germinal band at this stage there is seen to be an irregular, 

 but usually continuous, layer of cells (mesoderm) lying immediately beneath the cylindrical 

 cells (ectoderm) of the band. Lying at short intervals from one another are seen large 

 amoeboid cells apposed to, or fused with, this layer of smaller mesodermic cells. These 

 bodies or yolk cells, which have come to the surface from the central yolk mass, are five or 

 six times as large as the mesodermic cells and appear to be in process of division. They 

 doubtless give rise to the smaller mesodermic elements. The latter are closely apposed 

 to the under surface of th • germinal band and so flattened that their long axes are parallel 

 to the surface of the band. 



I am at present unable to affirm or deny the existence of the invagination 1 which in 

 most insects leads to the formation of these mesodermic elements, for, although 

 most of the facts relating to this mesodermic invagination s^eem to point to the conclusion 

 that it is wanting altogether, there are in some sections structural details indicating the 

 possibility of its occurrence in a modified form. The evidence against its occurrence in 

 Oecanthus is as follows : In sections of the egg in which the serosa is continuous over the 

 germinal band, and the amnion is present only in the form of a few flattened cells pressed 

 closely against the outer edges of the band, the mesodermic elements are seen in respective 

 sections to form either an unbroken line of small cells across the band, or two lines of 

 cells, one on each side the point at which the invagination . would occur. In some cases 

 only two, three or four cells occur at irregular intervals in the extent of the section. Here 

 and there the large yolk cells are fused with the under surface of the germinal band ; in 

 some cases they are apparently breaking up into cells of the size and appearance of the 



1 Compare Ticliomiroff (40). 



