182 



FORMATION OF THE LAYFU.S. 



increase in number and become one by one columnar; and that 

 in the process they press against the layer of rounded elements 



Fig. 136. Section through the nearly circular embryonic area of a Babbit's 



OVUM OF six days, NINE HOURS and -8 MM. IN DIAMETER. 



The section shews the peculiar character of the upper layer with a certain number 

 of supeificial flattened cells ; and represents about half the breadth of the area. 



below them, SO that the two layers cease to be distinguishable, and the 

 whole embryonic area acquires in section the characters represented 

 in fig. 13G\ Towards the end of the sixth day the embryonic area 

 becomes oval, but the changes which next take place are not under- 

 stood. In the front part of the area only two layers of cells are found, 

 (1) an hypoblast, and (2) an epiblast of columnar cells probably 

 derived from the flattened epiblast cells of the earlier stages. In 

 the posterior part of the blastoderm a middle layer is present (Van 

 Beneden) in addition to the two other layers ; and this layer probably 

 originates from the middle layer which extended throughout the area 

 at the beginning of the fifth day, and then became fused with the 

 epiblast. The middle layer does not give rise to the whole of the 

 eventual mesoblast, but only to part of it. From its origin it may 

 be called the bypoblasti c mesoblast , and it is probably equivalent to the 



A. B. 



Fig. 137. Views of the blastodermic vesicle of a Eabbit on the seventh 

 DAY without the ZONA. A. from above, B, from the side. (From Koliiker.) 

 ag. embryonic area; ge. boundary of the hypoblast. 



1 The section figured may perhaps hardly appear to justify this view; the exami- 

 nation of a larger number of sections is, however, more favourable to it, but it must 

 be admitted that the interpretation is by no means thoroughly satisfactory. 



