ON DEVELOP:\rEXT ETC. OF PITOROXIS. Ô21 



of course contain cacli a distinct nucleus. Sonic other cells are 

 apparently in tlie process of being pushed out and have a cluh-likc 

 shape, the narrowed end heini;' still inserted between the cells 

 of the layer to he invaginated. A further symptom of incipient 

 invagination consists in the circumstance that the nucleus in uK^st 

 cells of this portion has no longer a peripheral jwsition, hut is 

 situated in the middle or rather nearer to the inner end of the 

 cells. Moreover, the nucleus is frequently met with in the form 

 of the karyolvinetic figure which shows that the cells are dividing 

 and increasing in numhei- in the layer to he invaginated. Tltr 

 celU ini>^]i€x} out into the hladoccele arc nothing else than me><ohlast 

 celh, so that it may he dated that the niesohJast-fornuition begins 

 siinuUaneoudy uùth the gadrulation. 



At the beginning of gastrulation we can thus distinguish two 

 parts in the blastoderm wall, riz., the mesentohlad and the ecto- 

 hlad. The former corresponds to the whole of the portion to be 

 invaginated, while the ectoblast forms all the remaining portion of 

 the embryo. The mesentoblast is com])Osed of large and irregular- 

 ly arranged cells, while the ectoldast is of taller cylindrical cells 

 regularly arranged in a single row (figs. 2<) a and 2(3 h). The 

 mesentoblast, as the name indicates, is destined to give rise to 

 both the entoblastic and mesoblastic elements. The characteristic 

 disposition of its constituent cells indicates its l)eing the source of 

 mesoblast proliferation . 



The mesoblast ])rolifcration becomes more and more accen- 

 tuated in activity as the gastral invagination gradually deepens 

 (see fig. 27), ])ut the mesoblast cells thus formed do not adhere as 

 a lining epithelium to the ectoblast, until what are called the ant- 

 erior diverticula have been formed on both sides of the blastopore. 

 The blastocœle, in which tlie mesoblast cells are at first loosely 



