284 EMBRYOLOGY 



sists substantially of a bilaminar cellular vesicle, between 

 the two layers of which are lodged on the ventral side the 

 mesodermal bands. The blastopore has become the per- 

 manent mouth, in the neighborhood of which a lip-like 

 thickening of the ectoderm makes its appearance. The 

 cells lying around the mouth appear to be of a contractile 

 nature, and accordingly execute swallowing movements, in 

 consequence of which the intestine becomes filled with the 

 albumen in which the embryo floats, and which serves it as 

 food. As a result of this nutrition the embryo becomes 

 more and more distended, and increases in volume. The 

 embryo in this condition may be compared with the free- 

 swimming larvm of other Annelida, especially as it bursts 

 the vitelline membrane at about this stage, and now floats, 

 as has been mentioned, in the albumen of the cocoon. The 

 larva-like appearance of the embryo is increased by the fact 

 that in Lumhricus trapezoides (according to Kleinenberg) 

 there is found a ring of delicate cilia, surrounding the 

 mouth and continuing into a ventral ciliated groove, which 

 extends in the middle line between the mesodermal bands. 

 Hatschek also found an adoral ciliated zone in the embryos 

 of Criodrilus. Farthermore Yejdovsky proved the existence 

 in Rhynchelmis of a paired head kidney, which Bergh also 

 found in Criodrilus. It consists of a long, semicircular tube, 

 the blind inner termination of which lies in the vicinity of the 

 mouth, whereas the external opening of the ciliated canal is 

 situated laterally at about the middle of the body. In view" 

 of all this, the embryos of the Oligochaeta appear to be 

 degenerate larval forms, which float free in the albumen of 

 the cocoon, and here feed independently. The absence of the 

 anus does not enter much into the question, for we see that 

 in many Trochophore larvae also — for example, in Psygmo- 

 branchus — the anus, and even the mouth, may be wanting in 

 the early part of its free existence (comp. Fig. 128, p. 280). 



The metamorphosis of the larva- like embryos into the 

 worm is accomplished principally as the result of the further 

 development of the mesodermal bands. These at first con- 

 tinue their growth forwards and surround the fore-gut, 

 which has been formed out of an invagination of the ecto- 



