BIOLOGY: GENERAL AND MEDICAL 



Holoblastic. eggs of higher animals, having formed 

 a blastula, next undergo a peculiar invagination through 



SECTION OF GASTRULA. 



Archenteron. ,<*m^ SECTION OF PLANULA.. 



Epiblast 

 Blastopore. 



a b 



FIG. 85. a, Section of gastrula; b, section of planula. (Masterman.) 



the ascent of the vegetative pole of the blastula layer, 

 until it comes into contact with the cells of the animal 

 pole. The invaginated blastula thus comes to resemble 

 a hollow ball one side of which has been pressed in 

 against the other. The result of the invagination is 

 that the embryo now consists of a double layer of cells, 

 surrounding a new cavity formed by the invagination, 

 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 ccelenterates, and indeed this diploblastic 

 larva is the general plan of development, a* 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- 



