Development ^/"Rotifer vulgaris. 141 



(in Griodrilus) was long since observed and described by 

 Hatschek *. 



Hatschek's fig. 1, pi. i. of the memoir cited, shows the cells 

 in question exactly in the same position, with regard to the 

 two primary lamellae, in which I have met with them in the 

 egg of Philodina roseola. In the latter object I could also 

 ascertain beyond doubt that the mesodermal rudiment takes its 

 origin in the form of a portion cut off from the hypoblast. 

 When Hatschek says of these mesodermal cells " that they 

 show a nearer relationship to the entoderm than to the ecto- 

 derm," it is correct and easily intelligible, as they stand in 

 no genetic connexion with the latter. 



By the evidence adduced by me the Rotatoria also show a 

 significant relationship to the larva of Polygordius (the tro- 

 chophora), of which we know that it possesses mesodermal 

 streaks of exactly the same cellular constitution as the embryo 

 of Philodina roseola. 



Very probably the presence of a similar mesodermal rudi- 

 ment would be ascertained also in Rotifer vulgaris, if we had 

 any means of tracing the hypoblast in this animalcule more 

 distinctly. It is, however, quite impossible in the case of 

 liotifer to ascertain anything about its further fate (after the 

 ectoderm has grown over it). I obtained no result even by 

 stainiug it (with methyl-green). 



As regards the subsequent development of the embryos of 

 Rotifer vulgaris, in the first place a cephalic and a caudal 

 portion are constituted by a transverse furrow which early 

 makes its appearance upon the ventral side. On the former the 

 delicate outlines of the rotatory organ very soon appear in the 

 form of two semicircular folds, which meet together in a 

 middle line, the future oesophagus. At the same time we 

 observe on the caudal (or pedal) portion some shallow emar- 

 ginations by which the subsequent superficial segmentation 

 of this section of the body is already expressed. 



In the early developmental stages of Potif'er not the least 

 trace of the internal organs is to be discovered ; but their 

 presence and their perceptibility certainly do not occur at the 

 same moment. For no doubt the intestine is already present 

 when the outlines of the pharynx and its denticulated jaw- 

 plates- begin to show themselves, which, as is well known, 

 happens rather early. That at this period we are still unable 

 to detect the digestive canal is probably due only to the fact 

 that the difference between its refractive power and that of 



* ' Studien iiber Entwicklungsgeschichte der Auneliden,' 1878, pp. 3 

 and 92 et seqq. 



Ann. & Mag. N. Hist. Ser. 5. Vol. xv. 11 



