THE TURBELLARIA 



29 



organs, outgrowths of the atrial walls known as " adenodactyli " 

 or " adenocheiri," according to their shape. 



The function of the uterus (see Bergendal) appears to vary ; 

 in some cases spermatozoa are found in it (it is a spermatheca, 

 Bdelluridae) ; in others, eggs have their cocoon deposited around 

 them ; in others, again, the cocoon is moulded in the atrium, or 

 even in the vagina (some land Planarians), but the uterus secretes 

 the substance which will harden to form the shell. 



Development. From four to twenty or even more egg cells 

 .are surrounded by several hundred amoeboid (v. Siebold, 1841) 

 yolk cells in each cocoon. Each egg cell undergoes development, 



a--- 



FIG. XIII. Plans of the Genital Atrium in Aquatic Triclads to illustrate the various 

 Relations of the Uterus. 



1. Gunda; the uterus is here a diverticulum of the egg duct, a, sperm duct; &, penis, 

 with ductus ejaculatorius ; c, atrium genitale ; c', antrum masculinum ; c", antrum femininum ; 

 d, atrial (genital) pore ; e, egg duct, with muscles around it, and frequently functioning as a 

 vagina ; the distal region receives gland cells, which appear to secrete the cocoon ; /, oviduct ; 

 {/, uterus ; h, its opening into the atrium, or to the exterior. 



2. In Planar ia, Polycdis, and many others the uterus opens into the atrium directly. 



3. In Uteriporus the uterus opens to the exterior by an independent pore at the side of 

 the genital pore. 



4.Syncoelidiuni presents a pair of independent uteri (</', g"\ each opening to the exterior. 



but, as in some Gastropod Molluscs and Oligochaeta, they do not 

 .all survive. 



The Triclads present a very peculiar phenomenon during 

 segmentation, in that the blastomeres resulting from the nearly 

 regular segmentation move apart from one another (Metschnikoff, 

 Ijima, and Hallez, 29), and lie in a fluid which appears to 

 result from this breaking down or fusion of the yolk cells (Fig. 

 XIV. 2, 3). 



The exact details of the formation of the layers are imperfectly 

 known ; but the blastomeres become differentiated into flat epiblast 

 cells which surround and enclose the yolk material (Fig. XIV. 4) ; 

 into hypoblast cells which are arranged in two groups, while the 

 blastocoele is occupied by wandering cells, mesoblastic in appear- 

 ance, some of which give rise to additional epiblast, others to the 

 brain, and others apparently to additional hypoblast cells ; the 

 pharynx, which is at first "simple," is employed for engulfing 



