DEVEIOPMENT OP THE PHTLACTOLJilMATOTJS POLTZOA. 499 



wliicli had been included within it is thus liberated, and enters on 

 its free life in the surrounding water. 



Nitsche compares this brood-capsule to the ooDcium or ovicell of 

 the marine Polyzoa, which is formed as an external bud on the 

 body-wall, and into which the fecundated egg passes in order to 

 escape finally into the sea. 



It appeared to me some years ago, when engaged in examining 

 the larvae of Alcyonella, that these were set free into the body- 

 cavity of the parent, whence they subsequently gained exit 

 by the destruction of the tissues. IN'itsche believes that in this 

 case they obtained access to the body- cavity by accidental rupture 

 of the brood-capsule. It is possible that Nitsche may be right in 

 this ; at all events, without an opportunity of controlling, by 

 further examination, my original observation, I do not desire to 

 insist on its accuracy. 



In a subsequent memoir * Nitsche traces the formation of the 

 bud in Alcyonella and points out in detail the parts which the 

 endoderm and ectoderm (the two germinal layers) take in the 

 formation of the tissues and organs. He had already shown that 

 the wall of the cystid or zooecium of Alcyonella consists of three 

 different layers besides the externally excreted ectocyst or cuti- 

 cula. These are an outer epithelium, an inner epithelium, and a 

 tunica muscularis lying between the two and consisting of a 

 structureless supporting membrane on which lie transverse and 

 longitudinal muscular fibres. 



The first indication of the polypide-bud shows itself as a sac- 

 like bulging inwards of the cystid wall. In this bulging the 

 tunica muscularis, however, takes no part, but seems to be 

 absorbed at the spot where the bud occurs. The polypide-bud 

 consists therefore at this stage of a two-layered cellular sac, whose 

 inner layer, bounding its central cavity, passes continuously into 

 the outer epithelium of the cystid wall, while the outer layer is 

 continuous with the inner epithelium of the cystid. 



Nitsche follows Metschnikoff in regarding the outer epithelium 

 of the cystid as the outer germinal layer or ectoderm, the inner 

 epithelium as the inner germinal layer or endoderm ; and if 

 we further regard the tunica muscularis as a middle germinal 

 layer or mesoderm, we may view the young polypide-bud as com- 



* " Untersucliungeu iiber die Knospung der Siisswasserbryozoen, insbesondero 

 der Alcyonella" Sitzungsberichte der naturforscbenden Gesellscbaffc zu Leipzig, 

 1874. 



