EXPLAXATIOX OF PLATE IT. 



cp. cpiblast. 

 m. mesoblast. 

 ////. hypoblast. 

 ci\ embryonic rim. 



n.al. cells formed around the nuclei of the yolk which have entered 

 the hypoblast. 



ii. Cell formed around nucleus of yolk. 

 mg. medullary groove. 



5 a. Section through the periphery of the embryonic rim of the blasto- 

 derm of which fig. 5 represents a section. 



6. Section through the embryonic rim of a blastoderm somewhat 

 younger than that represented on PL ti. fig. B. 



7. Section through the most projecting portion of the embryonic riui 

 of a blastoderm of the same age as that represented on PI. vi, fig. B. 

 The section is drawn on a very considerably smaller scale than that on 

 PI. III. fig. 5. It is intended to illustrate the growth of the embryonic 

 rim and the disappearance of the segmentation cavity. 



7 a. Section through peripheral portion of the embryonic rim of the 

 same blastoderm, highly magnified. It specially illustrates the formation 

 of a cell {c) around a nucleus in the yolk. The nuclei of the blastoderm 

 have been inaccurately rendered by the artist. 



8 «, 8 ?), 8 (T. Three sections of the same embryo. Inserted mainly to 

 illustrate the formation of the mesoblast as two independent lateral masses 

 of cells ; only half of each section is represented. 8 « is the most posterior 

 of the three sections. In it the mesoblast forms a large mass on each side, 

 imperfectly separated from the hypoblast. In 8 6, from the anterior part of 

 the embryo, the main mass of mesoblast is far smaller, and only forms a 

 cap to the hypoblast at the highest point of the medullary fold. In 8 c a 

 c.ip of mesoblast is present, similar to that in 8*, though much smaller. 

 'Ihe sections of these embryos were somewhat oblique, and it has unfor- 

 tunately happened that while in 8« one side is represented, in 8& and 

 8 c the other side is figured, had it not been for this the sections 8& and 

 8 c would have been considerably longer than 8 a. 



9. Longitudinal section of an embryo belonging to a slightly later 

 stage than B. 



This section passes through one of the medullary folds. It illustrates 

 the continuity of the hypoblast with the remaining lower layer cells of the 

 blastoderm. 



