176 EMBRYOLOGY 



General Considerations. 



In considering the development of the Turbellaria, the first thing to 

 attract attention is the radial structure of the embryonic fundament : the 

 four large blastomeres from which, above and below, the entoderm cells 

 have separated, the radially arranged ectoderm cells, but especially the 

 four groups of mesoderm cells. This condition points to the affiliation 

 of the Turbellaria with radially constructed animals, a relationship which, 

 in fact, has been advocated, either on anatomical or embryological grounds, 

 by numerous writers (Kowalevsky, Selenka, Lang, Chun, Goette). An 

 attempt has been made to trace the Turbellaria back to the Ctenophora. 



convex shape and enclose the archenteric fissure between them. Before the 

 overgrowth of ectoderm is completed the two entoderm cells and the two 

 mesoderm cells have each divided into two. The blastopore shifts (about 

 ninety degrees) from the basal side to one end of the sltort secondary 

 axis, and becomes the i^ermanent mouth. Thus the pole of the blasto- 

 pore, which is also that of the polar cells ('?), becomes the caudal end of 

 the worm. At the opposite pole, which protrudes somewhat, a bundle 

 of clear cells represents the first trace of the frontal organ, while two 

 slight depressions, symmetrically placed and of short duration, are ac- 

 companied by i^ermanent thickenings of the ectoderm, which constitute 

 the fundament of the nervous system. A marked feature of the early 

 stages is the bilateral symmetry, which is noticeable from near the 

 beginning of cleavage up to the time the young become ciliated. " There 

 is no hint of a radial appearance in either blastula or gastrula." 



The only representative of the Rhahdoccela studied by Pekeyaslawzewa 

 was Macrostoma histrix, in which the eggs are so opaque that it is diffi- 

 cult to trace the early stages. After the egg is laid it undergoes contrac- 

 tions which result in elevations and deep excavations of all parts of its 

 surface. Several (four to eight) polar cells are formed at different points 

 on the periphery ; these have the appearance of very hard and compact 

 fusiform bodies (!). The first cleavage furrow divides the egg into two 

 unequal cells ; the second furrow, perpendicular to the first, divides each 

 of these into two equal parts, so that there are now two large and two 

 small blastomeres. The subsequent cleavage stages were not followed 

 accurately, but "it is certain that the two small segments representing 

 the ectoderm arise from two large segments." Gastrulation is by epiboly. 

 It is uncertain whether the blastopore becomes the permanent mouth- 

 opening, or is closed, and a new mouth formed at the cephalic pole. In 

 later stages the embryo is more transparent. The mesoderm then exists, 

 it is maintained, in the form of two longitudinal bands, each of which is 

 separated into two layers by a fissure : the body cavity. The outer layer, 

 uniting with the ectoderm, forms the subcutaneous muscular layer; the 

 inner, uniting with the entoderm, forms the muscular layer of the in- 

 testinal wall. Other organs are formed as in Aphanostoma. — The 



TUANSLATOKS.] 



