FORMATION OP GERMINAL LAYERS IN OHELONIA. 21 



ferent from fig. 8 where the ectoblast cells, although con- 

 tinuous in the median line with the mass below, still maintain 

 their columnar shape over the whole dorsal surface and thus 

 give an impression of the ectoblast extending entirely across ; 

 the ectoblast is in this section fused into the median mass 

 without retaining the slightest trace of the columnar arrange- 

 ment, and the median mass of cells thus expose their surface 

 to the exterior for a short space in the axial line [yk.p.). We 

 wish to emphasise the fact that this part directly behind the 

 blastopore is neither at this nor at any subsequent time until 

 considerably later (if ever at all), covered by the ectoblast of 

 the general surface of the body. This area we consider to 

 be the remnant of the yolk-plug of Rusconi found in 

 the Amphibian embryos. This will become clear in the 

 later stages. From the axial mass, where the layers are indis- 

 tinguishable, there extends toward each side a thick meso- 

 blastic wing under the ectoblast. The yolk seems to be dis- 

 tinct from the mass above, although, throughout this region, 

 protoplasmic threads seem to connect the two. 



Fig. 10 passes just in front of the dorsal lip of the blastopore 

 where the ectoblast reflects downward and forward to become 

 continuous with the axial strip of the entoblast or chorda- 

 entoblast (compare fig. 16). The blastoporic passage [bl.p.) 

 seen as a transverse space is still open on the left to the 

 exterior. The floor of the passage is formed "by a mass of cells 

 continuous with the yolk-plug ; in fact we may consider this a 

 part of the plug. At a lower level the mesoblast [mes.) 

 stretches out laterally as two wings from the median mass. 

 The relations of the yolk are the same as in fig. 9. 



So far the sections seem to have passed through the part 

 known as the " sickle." 



The next section represented (fig. 11) evidently passes 

 through what may be called the neck or isthmus, i. e. the point 

 from which the three prongs of the trident referred to in the 

 surface view radiate (compare fig. 1 b). Accordingly, the 

 entoblast is found only in the median line as a thickening con- 

 stituting the walls of the blastoporic passage [bl. p.), which is 



