210 BIOLOGY: GENERAL AND MEDICAL 



while the original segmentation cavity has virtually 

 been extinguished. The embryo is now called a gas- 

 trula, and is diploblastic, because it consists of an outer 

 layer of cells, the ectoderm or epiblast, and an inner 

 layer, the entoderm or hypoblast. The cavity formed by 

 the invagination is now known as the archenteron, and 

 becomes more and more enclosed by increase in the 

 cellular layers until the original bell-shape gives place to 

 a more spheroidal form with a central opening, the 

 blastopore. 



Embryos at this stage bear a distinct resemblance to 

 certain larvae of c relent er at es, and indeed this diploblastic 

 larva is the general plan of development, as well as the 

 foundation of structure of the medusae. 



Holoblastic eggs of still higher animals next progress 

 to the formation of triploblastic larvae through the forma- 

 tion of a third cellular layer, the mesoderm or mesoblast, 

 composed of slightly differentiated cells, which arise from 

 two endodermal rudiments symmetrically arranged on op- 

 posite sides of the central axis. In different embryos its 



FIG. 86. Section through the germ disc of a freshly laid fertilized hen's egg. 

 fh, Cleavage-cavity; wd, white yolk; vw, lower cell layer; dw, upper cell layer of 

 the blastula. (After Duval.) 



appearance and arrangement vary, but it is described by 

 Masterman thus: " It consists of a more or less complex 

 double layer of cells of which the outer layer lines the 

 epiblast and the inner covers the hypoblast. These two 

 layers enclose a spacious cavity, called the ccelum, which 

 is usually filled with a nutrient fluid. The ccelum is not 

 usually continuous, but it may be divided in the median 



