DIGYEMIDJE. 109 



very similar to that of an ordinary ovum. It divides first into two 

 and then into four approximately equal segments. Of the four seg- 

 ments one, however, remains passive for the remainder of the develop- 

 ment. The other three divide and arrange themselves so as partially to 

 enclose in a cup-like fashion the passive cell (fig. 61 A). The six 

 cells resulting from their division again 

 divide, giving rise to twelve cells, which 

 nearly enclose the passive cell, leaving only 

 a small aperture at one point. The whole 

 process by which the central cell becomes 

 enclosed is, as E. van Beneden points out, 

 identical with a gastrula formation by 

 epibole, and the space where the central 

 cell is left uncovered is the blastopore. A. 



The central cell itself gives origin to the Fm 61 A . GASTEULA STAGE 

 hypoblast cell of the adult, and the peri- OF DICYEMA TYPUS. B. VEKMI- 

 pheral cells to the epiblast. FOBMEMBBTOOFDIOTEMA.TTPUS. 



By this time the embryo has assumed Se^f 12 ^ 1 "' afterE ' van 

 an oval form, and the blastopore is situated 



at the pole of the long axis of the oval where the cephalic enlargement 

 is eventually formed. 



The subsequent development consists mainly in the closure of the 

 blastopore, and an increase in the number of the epiblast cells. Before 

 the development is completed, and while the embryo is still in 

 the body of the parent, two germs, destined themselves to give rise to 

 fresh embryos, appear in the hypoblast cell, one on each side of the 

 nucleus (fig. 61 B). The embryo continues to elongate, while 

 the anterior cells become converted into the polar cells. Cilia 

 appear simultaneously over the general surface, and the embryo 

 makes its way out of the body of the parent, usually at the cephalic 

 pole, and becomes itself parasitic in the renal organ of the host 

 in which it finds itself. At the time of birth the embryo may contain 

 a number of germs and sometimes even developing embryos. 



Infasoriform Embryos. The infusoriform embryos are capable 

 of living in sea-water and almost certainly lead a free existence. 

 In their most fully developed condition so far known they have 

 the following rather complicated structure (fig. 62 D, E, F, G). 



The body is somewhat pyriform, with a blunt extremity which is 

 directed forwards in swimming, and a more pointed extremity directed 

 backwards. The former may be spoken of as the anterior, and the 

 latter as the posterior extremity or tail. At the anterior extremity are 

 situated a pair of refractive bodies (r) which lie above an unpaired 

 organ which may be called the urn. 



The structure of the urn, the refractive bodies, and the tail may be 

 dealt with in succession. 



The urn consists of three parts: (1) a wall (u\ (2) a lid (I), and 

 (3) contents (gr). The wall of the urn is hemispherical in form, and 

 composed of two halves in apposition (fig. F). Its concavity is 



