ON DEVELOPMENT ETC. OF rilOllONIS. ol9 



deepest portion may eoiiveiiientlv l)e called the central depression. 

 Fig. 6 represents the ventral view uf an enil)ryo in which the 

 invagination has 1)eeome visil)le from the ontside, the central 

 depression being most deeply shaded in the iigure. In a slightly 

 more advanced stage, as the original wide depression grows deeper, 

 the external ojK'ning is gradnally drawn together and at a certain 

 stage (lig. 7) Itecomes transformed into an ahiKtst triangular 

 1)lastopore situated at a j)osition sHghtly anterior to the centre, as 

 was the case with the central depression. The anterior side of 

 the triangular l)lastopore is S(jniewhat ronnded by curving uniforndy 

 outwards, while posteriorly the two other sides gradnally approach 

 each other so as to meet at a point which may be called the apex 

 of the triangle. Leading backwards from this apex, there runs in 

 the mediaii line the so-called ])rimitive groove. This latter and 

 also the triangular shape of the blastpore are occasioned, in my opi- 

 nion, sim})ly by the blastopore, originally broadly open, becoming 

 narrowed by the special activity in the lateral posterior parts of 

 its posterior section. In other words, the cell-multiplication of the 

 ectoblastic layer , is carried on especially in the last mentioned 

 parts, so that there the i)ressnre, which is exercised by the ecto- 

 blast towards the invaginated layer, is more marked than in the 

 anterior and lateral Ijordcrs of the blastopore. As the result of 

 the alxn'e phenomenon, the deHnitive blastpore is pushed further 

 anteriorly, and consecjuenth', the archenteric cavity deepens in the 

 posterior direction, as shown in fig. 29. The above consideratioji 

 is snpported by the resnlts of actual measnrements of the size of 

 the embryos concerned. In spite of the fact that the embryo has 

 developed consideraldy the body-length does not show any signi- 

 ficant increase, remaining all the while at about (J. 12 mm. on an 



