Fig. 1. 
(94) 
PLATE XLII. 
P. oblongus. Sections drawn at intervals. Young ring stage cut transversely into sixty- 
three sections (.0075 mm.). [be = blastocoele; ep = epidermis; pb = periblast]. 
Diagram 12 = longitudinal median section, constructed from the transverse sections. .0075 
mm. X 280 equals apparent thickness of each section. This amounts to 2.1mm. 2.1 X 63 
gives apparent length of the cap, which is 132.3 mm. Measuring off 132.3 mm., I divided it 
into sixty-three equal parts, and then by vertical measurements of each section constructed 
the diagram. Thickness of upper layer in the sections. (Apparent of magnified 280). 
8th section = 15 mm. 27th section = 11.5 mm. 
10; neo Ou. fs) ee se Soll. es 
12 “ee ia — 15 “ee 29 “ a — 10 ae 
ils} e ee 30 us = it) f 
as oe la <* 3ist oF = (0) 7 
iy eel nes 32d sO U: 
Gites 3 = ib} 33rd ff = 10 s 
ily i =a, ye 34th sa = 9 os 
See ws ss tigy Sees ey = 8 
LOI: . ty Cee 36-50th “ =8 SS 
20) s =14 “ 53d uo = 10 
21st eS =13 “ 54th - = 9.5 
22d Ge Sao fay ot oO He 
osrd) Sag See. = Oe 
Dia = 12) Boe Ome 
25 _ = ip & 60“ vs = 8 a 
ope soo 138 Glstueee smog 
The second section takes in many epiblast cells, which are seen partly in surface owing to the 
convexity of the cap near margin. 
Nearly all the cells are in some stage of division throughout all the sections. 
Notice that the nuclei of the periblast are few in section two, more in three, most in five and 
are fewer towards the middle of cap (37th). They are more numerous beneath the embryonic 
fold than elsewhere under the ring. 
The lower layer becomes distinct only in section five, where we see not only a line of division, 
but notice that the nuclei retreat from this line somewhat. 
The lower layer is thickest (34 cells deep) in the embryonic fold; elsewhere the ring where 
it is well begun is 1-2 cells deep. The ring is seen as a true infolding in the thirty-second 
section and especially in the thirty-fifth. In some points the ring is scarcely begun, 7. e. there 
is no distinet ring. The axial portion of the ring ceases with the tenth section, being thus less 
than + of the entire length of the cap. 
The upper layer is thickest over the embryonic apical fold, being here from 4—6-cells deep. 
This thick area thins off gradually beyond the twenty-fifth section (7. e. between twenty-fifth 
and thirty-fifth). From the thirty-fifth onward the diminution in thickening continues but 
in a much less marked degree, dwindling down to about half the thickness of the embryonic 
fold. 
The thickness of the upper layer is nearly uniform in all transverse sections from margin to 
margin. 
The upper layer is composed of an epidermis and a deeper neural layer (neural at least in its 
apical region). The epiblast is composed of flattened cells that abut against the highest angle 
of the periblast. In no case does the ring appear to arise by an infolding of this layer; on the 
contrary, there is sometimes a small space left between the marginal epiblast cell and the nearest 
cell of the ring (so in 27th). The deeper neural layer, however, bends into the infolding lower 
layer as shown in the twenty-seventh, thirty-second, thirty-fifth, and thirty-seventh. The 
nuclei may be seen in all stages of division in the ring region. 
The number of cells in a section of the ring varies from two (32nd) to four (27th, 37th). In the 
floor, the ring is one cell deep (except in apical region — see above) or sometimes two cells deep 
(most often at the inner edge). Later it is plainly one cell deep. 
