EXPLANATION OF PLATE IIL 



ep. epiblast. 



II. lower layer cell. 



in. mesoblast. 



hy. hypoblast. 



sc. segmentation cavity. 



es. embryo swelling. 



11. nuclei of yolk. 



1. Longitudinal section of a blastoderm at the first appearance of the 

 segmentation cavity. 



2. Longitudinal section through a blastoderm after the layer of cells 

 has disappeared from the floor of the segmentation cavity. Ijcl. large cell 

 resting on the yolk, probably remaining over from the later periods of 

 segmentation. Magnified 60 diameters. (Hardened in chromic acid.) 



The section is intended to illustrate the fact that the nuclei form a 

 layer in the yolk under the floor of the segmentation cavity. The roof of 

 the segmentation cavity is broken. 



•2 a. Portion of same blastoderm highly magnified, to show the characters 

 of the nuclei of the yolk rt! and the nuclei in the cells of the blastoderm. 



2&. Large knobbed nucleus from the same blastoderm, very highly 

 magnified. 



2 c. Nucleus of yolk from the same blastoderm. 



3. Longitudinal section of blastoderm of same stage as fig. 2. (Hard- 

 ened in chromic acid.) 



4. Longitudinal section of blastoderm slightly older than fig. 2. 

 Magnified 45 diameters. (Hardened in osmic acid.) 



It illustrates (1) the characters of the epiblast; (2) the embryonic 

 swelling ; (3) the segmentation cavity. 



5. Longitudinal section through a blastoderm at the time of the first 

 appearance of the embryonic rim, and before the formation of the medul- 

 lary groove. Magnified 45 diameters. 



