218 PROCEEDINGS OF THE ACADEMY OF [Feb.. 



meres form the embryonic pharynx; others wander through the syn- 

 citium. 



4. The embryonic layer which covers the secondary yolk (if) takerv 

 in by the embryonic pharynx, in no way corresponds to the ordinar}- 

 gastrula-stage. The sohd embryo has, by sucking in yolk through it-- 

 phar3mx, become a hohow ball filled with secondary yoke-cells (i/). 

 The embryo now consists of a single layer of spicitial yolk-material, 

 containing scattered blastomeres which feed on the primary yolk- 

 material and multiply until they occupy the whole space previoush' 

 filled by the primary yolk (figs. 43-46). Then the inner embryonic 

 cells begin to serve as endoderm-cells to absorb the secondary yolk. 



5. The axial gut and its principal branches are formed by ingrowths 

 from the embryonic layer, dividing up the central space which is filled 

 with secondary yolk (y^). 



6. Ectoderm, endoderm, permanent pharynx, eyes, nervous system, 

 reproductive organs, gland- and muscle-cells are ah formed by direct 

 differentiation of the embryonic cells of the ojie embryonic or germ- 

 layer. There is no formation of two or three distinct germ layers, 

 nor are anj- of the organs formed by folding as in most other forms. 



Literature, 



Curtis, W. C. '00. On the Reproductive Sj^stem of Planaria slmplissima, :-. 

 new species. Zool. Jahrb., Bd. 13, 1900. 



'02. The hfe history, the normal fission, and the reproductive organs o;" 



Planaria maculata. Proc. of Boston Soc. of Nat. Hist., Vol. 30, 1902. 



Hallez, p. '79. Contributions a I'histoire naturelle des TurbeUaries. Tra'-. 



de Vinstitut zool. de Lille, Fasc. 2, 1879. 

 Ijima, I. '84. Untersuchungen iiber den Bau und die Entwicklungsgeschichte 



der siisswasser-Dendrocoelen (Tricladen). Zeit. f. wiss. Zool, Bd. 40, 1884. 

 Kennel, J. VON. '79. Die in Deutsclilandgefundenen Land-planarien, u. s. w. 



Arb. Zool. Inst. Wiirzburg, Bd. 5, 1879. 

 Metschnikoff, E. 'S3. Die Embryologie von Planaria polychroa. Zeit. j. 



iviss. Zool., Bd. 38, 1883. 

 Schmidt, 0. '59. Die dendrocoelen Strudelwiirmer aus der Umgebung von 



Graz. Zeit. /. u't.ss. Zool., Bd. X, 1859. 



'61. Untersuchungen liber Turbellarien von Corfu u. Cephalon. Ibid.. 



Bd. XI, 1861. 



'61. Uber Planaria torva, auct. Ibid. 



Stevens N. M. '01. Notes on regeneration in Planaria lugubns. Arch. f. 

 Entwickelungsmech., Bd. VIII, 1901. 



Description of Plates XIII, XIV, XV axd XVI. 



Plate XIII, Fig. 1. — Ovary showing large oocytes and oviduct full of sperma- 

 tozoa (od) Bausch and Lamb, obj. ^ in., oc. C, camera. 



Fig. 2, a and b. — Ovarian egg showing first maturation-spindle with S 

 chromosomes, y = yolk granule. B. and L. -^-C. 



Fig. 3, a and b. — Same as above with 6 daughter chromosomes. 



