FORMATION OF GERMINAL LAYERS IN CITELONIA. 217 



the region of the " sickle," and there is no longer a mesoblastic Aving 

 on each side. 



Fig, 12 passes through the posterior part of the loAver opening 

 of the blastoporic passage. The tliickenings which form the lateral 

 walls of the opening are therefore still quite thick (compare fig. Ih). 

 The columnar chorda -entohlast is found as before forming the roof 

 and the sides of the passage, which is now open below. Towards the 

 lower part of the side walls the columnar arrangement is lost and the 

 cells are irregularly scattered. Further out at the sides the cells form 

 a loose network, and then at the edge of the embryonic shield passes 

 into the yolk. 



Fig. 13 passes through the anterior part of the ^'entrai opening 

 of the blastoporic passage, which has now flattened itself out into a 

 shallow groove in the median line. Its roof is still formed by the 

 distinctly columnar chorda-entoblast. Laterally, the chorda-entoblast 

 gradually passes into a mass of cells arranged in an irregular loose 

 network, which in its turn is replaced by the yolk at tlie edge of the 

 embryonic shield. 



Passing forward, the chorda-entoblast begins gradually to confine 

 itself more and more to the ventral median surface, until in the 

 seventh section from fig. 13 it has the appearance presented in fig. 14. 

 Here the columnar shape is confined to a few cells in the ventra^ 

 median line. They pass above gradually into the loose network of 

 cells which has now extended itself entirely across. The meshes of 

 the network have also become larger than in the previous sections. 

 It is evidently this loose network that produced the appearance of a 

 honeycomb in fig. 16. 



Fig. 14 passes in front of the ventral opening of the blastoporic 

 passage, and indicates that the loosely scattered lower layer cells are 

 here arranging themselves into the chorda-entoblast in the ventral 



