THE DKVKLOl'MKNT OF PALUDINA. 



135 



Meanwhile, by the development of two rows of large ciliated 

 ectoderm-cells, placed transversely to the gastrula-axis, the Trocho- 

 phore stage is reached. The pre-oral ciliated ring thus borders 

 the cephalic area, which has become larger by the increase in 

 number of the cells (Fig. 56 A). The blastopore marks the posterior 

 end of the embryo, but in consequence of somewhat stronger growth 

 and the consequent bulging of the ventral surface it is shifted slightly 

 dorsally. The blastopore is said to be retained in Paludina and to 

 pass over into the anus (Butschli, v. Eklanger). It has, however, 

 been asserted that the blastopore closes (Rabl) and that the mouth 

 and the anus are only indirectly related to the primitive mouth, as 

 we shall describe later (c/. p. 141).* A large, somewhat sunken 



a 



V. 



•nw*.- 



v. 



- - ma> . 



■A.-- 



--T1U&. 



FlG. 56. — .1 frontal and 1> sagittal section of two embryos of Paludina of different 

 ages (after Tonniges). m, region where the mouth develops at a later stage ; mes, 

 mesoderm -bands (in A) and scattered mesoderm-cells (in B) ; sd, shell-gland; nd, 



archenteron ; r, velum. 



area, which lies dorsally in front of the blastopore and consists of 

 columnar ectoderm-cells (Fig. 56 B, cd), represents the shell-gland, 

 above which the chitinous shell soon appears. An ectodermal de- 

 pression (in) which appears on the ventral side behind the ciliated 

 ring, ami which becomes connected later with the archenteron, yields 

 the stomodaeum. At this stage, the anterior part of the embryo has 

 lost its former bell-shape and has become more flattened (Fig. 56 B). 

 The mesoderm has lost its regular arrangement and has become for 



* [Tonniges (No. XXV.), the most recent investigator of the development of 

 i'uludina, finds that the oval blastopore closes from before backward, and 

 that it does not give rise to the anus, which, as a secondary formation, appears 

 at the point where the blastopore closes. — Ed.] 



