44 



FORMATION OF MESOBLAST. 



shallows out as it extends inwards. This groove is the me- 

 dullary groove ; and its termination at the edge of the blasto- 

 derm is placed at the hind end of the embryo. 



At about the time of its appearance the mesoblast becomes 

 first definitely established. 



At the edge of the embryonic rim the epiblast and lower 

 layer cells are continuous. Immediately underneath the me- 

 dullary groove, as is best seen in transverse section (fig. 18), the 

 whole of the lower layer cells become converted into hypoblast, 

 and along this line the columnar hypoblast is in contact with 

 the epiblast above. At the sides however this is not the case ; 

 but at the junction of the epiblast and lower layer cells the 

 latter remain undifferentia- 

 ted. A short way from the 

 edge the lower layer cells 

 become divided into two dis- 

 tinct layers, a lower one con- 

 tinuous with the hypoblast 

 in the middle line, and an 

 upper one between this and 

 the epiblast (fig. 18 B). The 

 upper layer is the commence- 

 ment of the mesoblast (m). 

 The mesoblast thus arises 

 as two independent lateral 



, . - FlG. l8. TWO TRANSVERSE SECTIONS OF 



plates, one on each side 01 AN EMBRYO OF THE SAME AGE AS FIG. 17. 



it.at 



A. Anterior section. 



B. Posterior section. 



mg. medullary groove ; ep. epiblast ; hy. 

 hypoblast ; n.al. cells formed round the 

 nuclei of the yolk which have entered the 

 hypoblast ; 111. mesoblast. 



The sections shew the origin of the 

 mesoblast. 



the medullary groove, which 

 are continuous behind with 

 the undifferentiated lower 

 layer cells at the edge of the 

 embryonic rim. The meso- 

 blast plates are at first very 

 short, and do not extend to the front end of the embryo. They 

 soon however grow forwards as two lateral ridges, attached to 

 the hypoblast, one on each side of the medullary groove (fig. 18 

 A, ;#). These ridges become separate from the hypoblast, and 

 form two plates, thinner in front than behind ; but still continu- 

 ous at the edge of the blastoderm with the undifferentiated cells 

 of the lip of the blastopore, and laterally with the lower layer 



