C24 KNOWER. [Vol. XVI. 



end of the disc. There are no separate lateral folds, nor is 

 there any "head-fold." In a series of specimens represented in 

 Figs. 19-24, Pis. XXX and XXXI, the membranes are found 

 extending further and further anteriorly over the disc. In 

 PI. XXX, Fig. 23, the amniotic cavity remains open in only a 

 single spot at the anterior extremity of the disc, the closure of 

 which opening, in PI. XXXI, Fig. 24, completes the process. 



A series of sagittal sections, like that shown in PI. XXXI, 

 Figs. 32-35, gives a better idea of what has just been pointed 

 out in the surface figures. (The nuclei in the resting stage in 

 this series of figures are represented in solid black for the sake 

 of clearness. They resemble those in PI. XXXI, Figs. 30 and 

 31, being large, vesicular, and containing fragmented masses of 

 chromatin.) 



PI. XXXI, Fig. 32, already referred to in a previous section, 

 exhibits the appearance and relations of the amnio-serosal and 

 mesodermal rudiments when first well established. The meso- 

 dermal collection of cells lies under the anterior half of the 

 embryonic disc. It does not extend beneath the extreme 

 anterior end, and is still rather intimately associated with the 

 ectoderm from which it arose. Behind this mesodermal plug, 

 and between it and a posterior thickening of the ectoderm 

 (already indicated as the first rudiment of the amnion), is a 

 thinned region of the disc with only one layer of nuclei, 

 corresponding to the lighter portion of the surface view in a 

 like position. Note the immensely enlarged yolk-cell nucleus 

 as compared with one of the mesoderm. 



In PI. XXXI, Fig. 33, a section of the stage (PI. XXX, Fig. 19), 

 except for an increase in the size of the rather loose mesodermal 

 plug (due partly to a continued migration from the ectoderm, 

 as indicated by the direction of the spindle of the dividing 

 ectoderm nucleus anteriorly, and by the crowding of the cells 

 in the lower layers of the ectoderm), the most striking change 

 is a bending forward of the thickening, marked amnion in the 

 preceding stage, to form a fold. The bend takes place in the 

 thin, single-layered portion of the disc. The serosal cell pos- 

 teriorly is much flattened, and is drawn forward by a very 

 slender thread of protoplasm. It is interesting to observe, in 



