ORIGIN OF THE ARCHENTERON. 253 



The coelomic plates ultimately form the somatic muscles, 

 and I am inclined to think that the inner plate (the Darm- 

 drusenblatt of the Germans) enters into the formation of the 

 muscles of the alimentary canal. 



Cholodkowsky has very carefully studied the development of the ccelomic 

 cavity in ' Blatta Germanica,' and arrives at the following conclusions, which 

 are quite consonant with the view I have given above of the changes which 

 the ccelomic sacs undergo in the Blow-fly embryo : 



1. 'In "Blatta Germanica" the body cavity is developed from eighteen pairs 

 of hollow somites, which originate from the segmentation of the epiblast 

 and coelomic sacs.' 



2. ' The cavities of the somites each divide, as in Peripatus, into three 

 parts, one of which is probably homologous with the segmental funnel.' 



3. 'The somatic cavities ultimately become broken up (aufgegeben), and 

 open into the periccelomic space, which corresponds with the segmentation 

 cavity, and is permeated by the parablast.' 



4. ' The ccelom, or body cavity, in its definitive stage, has therefore a three- 

 fold origin ; it consists of the primitive cavities of the somites (/.., the 

 cavities of the segmented ccelomic sacs), of the primitive segmentation 

 cavity, and of a true schizocoele, which is subsequently formed.' 



I am unable to trace any appearance of a true schizocoele in the Fly 

 embryo, but think it probable that only a small portion of the inner ccelomic 

 plate enters into the formation of the alimentary canal. If this is so, a 

 schizocoele may be assumed to exist, although the invasion of its cavity by 

 spongy parablast masks its true character. 



5. 'The heart is developed from the primitive segmentation cavity.' I 

 think this statement should be : the segmentation cavity forms the pericardium, 

 not the heart. 



6. ' The fat bodies are derived from yelk cells, which at a certain stage are 

 amoeboid, and wander into the body cavity.' This is merely another form of 

 the statement that the fat bodies are of parablastic origin. 



7. With regard to the origin of the ccelomic sacs in Blatta, the memoir 

 quoted gives little information, but at the moment of going to press I 

 received a much-extended memoir of Cholodkowsky's on the development 

 of Blatta Germanica. f In this paper he states that the mesoblast (his inner 

 germinal layer) is derived from an invagination which takes place in the 

 ventral median line, the primitive trace (Primitivrinne). I think that the 

 drawings given by Cholodkowsky of the primitive trace, which are accurate 

 representations of sections, do not bear the interpretation he puts upon them. 

 In this I am in unison with Tichomirovv, who denies that the primitive trace 

 has anything to do with the formation of the mesoblast. 



* Cholodkowsky, N., ; Zur Embryologie von Blatta Germanica,' Zool. 

 Anzeig., Bd. xiii., p. 137, 1890. 



t Cholodkowsky, ' Die Embryonalentwicklung von Phyllodromia (Blatta) 

 Germanica,' Mem. de 1'Acad. Imp. des Sc. de St. Petersbourg, torn, xxxviii. 

 No. 5, 1891. 



