SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 97 



elements that, with the appropriate hereditary influences, might be 

 fashioned into a flatworm, an annehd, an arthropod, a moUusk, or 

 a vertebrate. 



The typical planula is a minute oval or elongate creature (fig. 

 I A, C) consisting of an outer layer of ectoderm cells, and an in^er 

 mass of endoderm cells. The planula, therefore, represents the 

 gastrula stage of embryonic development, though it may have no 

 enteric cavity and no blastopore. Its motor mechanism is a covering 



Fig. I. — The coelenterate planula, and two methods of endoderm formation. 



A, planula of Sympodium corraloidcs (from Kowalevsky and Marion, 1883). 

 B, blastula of Carmarina fungiformis, showing dififerentiation of endoderm from 

 ectoderm by delamination of blastoderm cells (from Metschnikoff, 1882). C-F, 

 formation of endoderm by internal proliferation from posterior pole of planula 

 (from Hatschek, 1888, after Claus). 



Blc, blastocoele ; Bid, blastoderm ; Ecd, ectoderm ; End, endoderm. 



of vibratile cilia. The embryology of the planula is very simple. The 

 cleavage of the coelenterate egg produces a morula, and the morula 

 becomes a blastula. In the succeeding planula stage the inner endo- 

 dermal cell mass is formed, but it is not certain that gastrulation 

 takes place by simple invagination in any of the coelenterates. With 

 some forms the endoderm arises as an inward migration of scattered 

 cells from the blastoderm ; in others the blastomeres divide regularly 

 each into an outer ectoderm cell and an inner endoderm cell (fig. i B) ; 

 but the most common method of endoderm formation is the internal 



