DEVELOPMENT OF SIMOCEPHALUS 641 
and anteriorly to this a mesendodermal mass of cells from which 
arises the mesendoderm. 
5. The genital rudiment surrounded laterally and posteriorly 
by inwardly growing ectomesodermal cells invaginates and 
becomes internal by the lips of the invagination growing 
together and fusing. 
6. The mesendoderm grows backwards as a solid mass of 
cells, which later spreads out flat and becomes indistinguishable 
from the laterally-lying mesoderm, and from this layer the 
endoderm separates as a solid rod in the median plane. 
SoutH KENSINGTON, 
= 
June 1921. 
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