310 SUMMARY OF CURRENT RESEARCHES RELATING 



and A. Schroder describe Myenchus bothryophorus g. et sp. n., which 

 occurs in Nephelis vulgaris, especially within the smooth muscle-cells. 

 It was also found in the connective tissue and in the cocoon of the same 

 leech. There is only one previous record of a Nematode parasitic in 

 leeches (in the body-cavity of Glossiphonia stagnalis or Clepsine oculata), 

 and the occurrence inside smooth muscle-cells is also remarkable. The 

 new form, which is marked by the possession of a ventral groove, comes 

 nearest to Tylenchiis and ApMlenchus, but neither of these genera has 

 any representative parasitic in animals. 



Platyhelminthes . 



Development of Planaria simplissima.* — N. M. Stevens finds that 

 in Planaria simplissima the division of the chromosomes in both 

 maturation divisions is longitudinal ; that the number of chromosomes 

 in the maturation divisions varies from three to six, but is usually three ; 

 that there is nothing corresponding to a typical blastula or gastrula ; 

 that after several segmentations the blastomeres form an irregular group, 

 embedded in a syncytial yolk-mass which forms a part of the embryo. 



Some of the blastomeres form the embryonic pharynx ; others 

 wander through the syncytium. The embryonic layer which covers the 

 secondary yolk taken in by the embryonic pharynx, in no way cor- 

 responds to the ordinary gastrula-stage. The solid embryo has, by 

 sucking in yolk through its pharynx, become a hollow ball filled with 

 secondary yolk-cells. It consists of a single layer of syncytial yolk- 

 material, containing scattered blastomeres which feed on the primary 

 yolk-material and multiply until they occupy the whole space previously 

 filled by the primary yolk. Then the inner embryonic cells begin to 

 serve as endoderm-cells to absorb the secondary yolk. 



The axial gut and its principal branches are formed as ingrowths 

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

 with secondary yolk. Ectoderm, endoderm, permanent pharynx, eyes, 

 nervous system, gonads, glandular cells and muscle-cells, are all formed 

 by direct differentiation of the embryonic cells of the one embryonic or 

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

 nor are any of the organs formed by folding, as in most other forms. 

 Altogether it is a remarkable story. 



Early Development of Fresh-water Dendroccelida.f — E. Mattiesen 

 confirms the results of earlier observers who held that the syncytium 

 .surrounding the blastomeres arises by the fusion of the yolk-cells. He 

 observed the process in Planaria torva. He further notes that the 

 embryonic mesenchyme contains elements of all the three germ layers, 

 which explains the origin of diverse organs from it. The development 

 has hardly anything in common with that of marine polyclads, which 

 retain primitive characters. The various modifications in the fresh- 

 water forms are mainly due to the development in the centre of a yolk- 

 cell mass. 



* Troc. Acad. Sci. Philadelphia, 1904, pp. 2C8-20 (4 pis. and 5 figs.), 

 t Zool. Anzeig., xxvii. (1904) pp. 81-87. 



