358 EMBRYOLOGY 



membrane, now begins to rotate. The vegetative pole, which 

 begins to flatten, and then to invaginate, remains free from 

 cilia. 



In the formation of the archenteron a small part of that 

 portion of the blastula which is still to be assigned to the 

 ectoderm (Fig. 158 B) is also invaginated. The boundary 

 between it and the entoderm is marked by the mesoderm, 

 which has increased to two cells (the primitive mesoderm 

 cells) and now moves inward. The two cells are symme- 

 trical in relation to the median plane. The depression of the 

 ectoderm already mentioned, which follows that of the 

 entoderm, gives the first impulse to the development of the 

 permanent larval skin. It sinks in deeper and deeper and 

 bends over forwards, forming in this way a lamella of thick 

 cells (Fig. 158 G and D, rp). The plate remains temporai-ily 

 united, by means of an amnion-like fold that is not exten- 

 sive, it is true, with the primitive ectoderm, which soon 

 appears only as the serosa (Fig. 158 D). Hatschek calls 

 the plate the trunk-plate in contrast to the head-plate, 

 which is also differentiated from the primitive ectoderm. 

 This differentiation takes place as follows : — In the region of 

 the animal pole (corresponding to the apical plate), which 

 has now also become thickened, the cell plasma retracts 

 from the egg-membrane in an annular furrow, and thus 

 gives rise to a circular space (Fig. 158 B to D, ka) be- 

 tween the permanent ectoderm and an outer layer (serosa). 

 The head -plate, therefore, corresponds to the apical plate. 

 The space between it and the serosa (the amnion is only 

 slightly developed here) Hatschek calls the head amniotic 

 cavity (ka), and that between the trunk-plate and serosa the 

 trunk amniotic cavity (ra). The fold w^hich would corre- 

 spond to the amnion on the trunk-plate is retained for a 

 short time and is then included in the formation of the 

 trunk-plate. The trunk- and head-plates alone supply the 

 ectoderm of the larva. All the rest of the ectoderm of the 

 embryo is employed in the formation of the embryonal 

 membrane (serosa). The serosa grows over the trunk-plate 

 and completely encloses it (Fig. 158 D and E, se) ; however, 

 this is not the case at the opposite (animal) pole. The 



