584 WILLIAM PATTEN. 



During the present stage the mesoderm increases in extent, and 

 at the same time the cells composing it become less definitely 

 and compactly arranged. The yolk, on account of its absorp- 

 tion by the growing embryo, becomes reduced in amount, and 

 gradually recedes from the ventral plate. Into the cavity thus 

 formed amcfiboid mesoderm cells migrate, and give rise to the 

 blood-corpuscles (PI. XXXVIb, fig. 20). As the yolk continues 

 to withdraw from the ventral plate the mesoderm follows it by 

 a rapid growth along its lateral margins, which, when they come 

 in contact with the yolk, are reflected backwards and inwards 

 towards the median line, thus forming, on either side of the 

 body, a mesodermic layer, which bounds the yolk on the ventral 

 side. 



By continued growth the margins finally meet each other. 



The mesoblastic wall, however, does not entirely separate the 

 yolk from the body cavity, for it is segmented in such a way 

 as to produce bands of mesoderm extending entirely across the 

 ventral side of the yolk, alternating with other bands, which 

 are interrupted in the middle line, thus allowing free passage 

 from the yolk into the body cavity. A section through one of 

 these interrupted bands is shown in PI. XXXVIb, fig. 20, where 

 one or two yolk-cells are represented in the act of migrating 

 from the yolk into the body cavity. 



In the meantime the lips of the mesoblastic folds have pro- 

 gressed in an opposite direction, as shown by arrow No. 1 (PI. 

 XXXVIb, fig. 20), giving rise to two layers of mesoderm, an 

 inner and an outer. The inner layer or splanchnic mesoderm 

 extends over and encloses the yolk, with its muscular layer 

 before the yolk or endoderm cells have formed the 

 epithelial lining of the stomach; the outer layer or 

 somatic mesoderm becomes closely united to the ectoderm, 

 with which it extends dorsally to form the outer wall of the 

 sides and dorsum. Before the folds have met in the middle 

 line of the dorsum- the amnion and serosa rupture, so that 

 the further consideration of the changes in these layers will 

 be deferred until the next stage. 



The endoderm cells increase very rapidly during this stage. 



