340 



LEWIS R. GARY. 



In Metridium, according to McMurrich, the result of segmenta- 

 tion is the formation of a hollow blastula with a considerable 

 cavity. Later the inner ends of the cells are constricted off- 

 by the appearance of vacuoles in the line where the separation 

 is to occur to form the endoderm. At the time when this proc- 

 ess is finished there appears at one pole of the blastula a slight de- 



FIG. i. Pseudogastrulation in Metri- FIG. 2. Later stage in the pseudogastrula- 

 diiim. After McMurrich. tion of Metridium. After McMurrich. 



pression which gives the embryo, when it is seen in optical section, 

 an appearance similar to the early stages of a true invagination 

 (Fig. i). 



When the mouth has broken through, the resemblance to an 

 invaginate gastrula is even more complete, so that until such 

 embryos had been seen in sections it would be almost certain to 

 mislead any observer. In reality, however, the two layered 

 condition had been readied before the- mouth was formed. This 

 so called "Pseudo-gastrula" McMurrich held to be the true con- 

 dition in those forms in which invagination had been reported 

 to occur. 



All the material of A. bermudensis was obtained by slitting 

 the adult individuals longitudinally, and then \\.i--l ling the em- 

 bryos into a dish of sea water with the stream from a pipette. 

 All stages including the young in which the second series of 

 tentacles was complete, and which wen- ready to be liberated 

 from the body of the parent were capable of swimming about 

 actively by means of their cilia. 



The earliest segmentation stages were never found among the 



