ox THE PROCESS OF GAS'J'RULATIOX IX CHELOXIA. 241 



is ever getting longer (Fig^. 14, 15, 16 & 17). In the anterior part 

 of the lloor of the invagination-ca^■ity whicli was already losing its 

 sharp definition in Fig. 14, tlie disrii]»tion lias proceeded une step 

 farther in Fig. 15. In this figure, not only this part of the fioor is 

 giving away, but the network of cells lying underneath it, and 

 between it and the subeferniinal volk cavitv, has been laroeh' absorbed. 

 In Fig. 16, the process of breaking through is seen to be complete, 

 and the invagination-cavity has now a clear opening below. I think 

 it almost certain that such a clear and comparatively large opening- 

 has been produced by the running together of se^'eral small openings, 

 such as we see in Figs. 14 and 15, which put the meshes of the cell- 

 network in communicati(Hi with the invagination -cavit}'. In fact, in 

 Fig. 16 we can still see several such openings in the floor of the 

 cavity in that part of the network situated behind the large anterior 

 opening and in front of the compact peristomal mesoblast mass. Com- 

 parison with Fig. 17 makes it probable that this part of the cell- 

 network is to be eventually absorbed, for the single large opening- 

 extends in the latter back almost to the peristomal mesoblast. It 

 should also be noticed in Fig. 16 that the extreme anterior end of the 

 invagination-cavity is clearly recognisable and does not particii>ate in 

 the breaking through, which seems to be confined to the floor. We 

 should therefore remember that although the anterior end may n<jt be 

 recognisable in later stages (e.g. Fig. 17), it is the floor which is 

 open below. The surface views of the stage at which the invagina- 

 tion cavity has just o])ened below are given in Figs, o his, 4, and 4«. 

 There is considerable difference in the appearance of the two embryos 

 Avhich I am not able to explain. I drew them just as they appeared 

 under the microscope. I am rather inclined to think that Fig. 4(a 

 represents a more normal appearance, if we are to judge from the 

 succeeding stages, although I am unable to detect anything unusual 



