XIXI 



EOTIFEEA 



423 



The next change which occurs is the invagination of the 

 rudiment of the primary stomodaeum. The layer of clear ectoderm 

 cells now forms a complete mantle all round the egg. The granular 

 cells divide and form a two-layered plate of cells lying obliquely 

 across the egg. Soon afterwards the outer clear ectoderm, at the 

 spot where the granular cells were covered over, is invaginated in the 

 form of a wide funnel, which is the secondary stomodaeum (Fig. 334). 



Soon afterwards the whole ventral surface becomes concave, and 

 in this way the rudiment of the tail, which occupies the hindermost 

 part of the concavity, is marked off from the rest of the body. 

 The front border of the concavity, which is at the same time the 



PiQ. 334. — Optical sagittal sections of two embryos of Callidina russeola after the process 

 of gastrulation has been completed. (After Zelinka.) 



A, earlier stage. B, later stage : the secondary stomodaeum is formed and the differentiation 

 between head and tail has begun, caitd, rudiment of the tail ; end, endodermal cells ; gr, granular 

 cells ; m, mouth ; stom?-, primary stomodaeum consisting of the granular cells ; stom^, secondary stomo- 

 daeum consisting of ordinary ectoderm. 



hinder border of the mouth, is the rudiment of the under lip. The 

 tail, once begun, grows rapidly forwards, so that the embryo is bent 

 double (Fig. 334). 



In front of the mouth an overhanging lobe of cells becomes 

 prominent; this is the rudiment of the apical plate out of which 

 both proboscis and trochal plate grow. It is kidney-shaped in 

 outline, its longest axis being transverse to that of the embryo, and its 

 concave border being in front. The concavity deepens until the plate 

 is obviously bilobed (Fig. 335). 



Meanwhile the endoderm cells have been undergoing division, and 

 they become grouped into two masses, the front one being derived 

 from the anterior four cells and the hinder from the posterior four. 

 The posterior mass of cells is contained in the tail or foot, whilst on 



