808 SUMMARY OF CURRENT RESEARCHES RELATING TO 



(d) Morphological differentiation of the endoderm. — Dr. Hcider 

 gives a most interesting account, accompanied with lucid diagrams, 

 of the formation of ciliated chambers from foldings of the endoderm. 

 Wo have first to noto the formation of flat diverticula, mostly towards 

 the upper or aboral surface. It is as if the endoderm forms over the 

 entire periphery of the aboral surface a prominent pad, which is very 

 variously interrupted in its course. The sacs which interrupt the 

 continuity of this pad are quite irregular in their distribution. Soon, 

 however, radial folds appear, pushing the peripheral pad inwards, 

 producing a zigzag pattern. The pad is further divided into separate 

 segments by cross folds, and these segments are again divided into 

 outer and inner portions. Two circles of small diverticula thus 

 result, and these are seen to be the rudiments of the ciliated chambers. 

 Without the illustrative diagrams it is, however, difficult to summarize 

 the differentiation of the folds. 



(e) Further processes till the closure of the blastopore. — The long 

 prismatic form of cell is wholly lost, and the yolk -like granules in 

 the endoderm have disappeared. The cells round the rudimentary 

 ciliated chambers are bottle- or pear-shaped ; the collars are even 

 more distinct ; exo- and endoplasm are readily distinguishable. The 

 other endoderm-cells round the large primitive gastral cavity are 

 cubical and colourless, but still ciliated. Between the ectoderm and 

 endoderm a middle layer of cells appears. This mesoderm seems 

 probably derivable from cells which have migrated from the endo- 

 derm ; at least, the facts that the first cells appear near the endoderm, 

 that the youngest cells are histologically very like the endoderm 

 cells, and that phenomena suggestive of migration were observed, 

 have led Dr. Heider to support this theory of the origin of the 

 mesoderm. The ectoderm cells are quite cubical, or sometimes even 

 flatter. The development of the processes which serve, probably 

 with the aid of some cementing secretion, to fix the larva, is described. 



(/) The definite formation of the ciliated chambers. — The gastral 

 cavity becomes distended with a watery fluid containing albuminous 

 substances in solution ; the flat larva increases in height and becomes 

 hemispherical ; the ectoderm becomes smoother, the smaller processes 

 disappear, and the cells become less cubical and more flattened. 

 More important changes, however, occur in the endoderm. The two 

 rows of diverticula take up a more lateral and less superior position, 

 the summit of the hemisphere is occupied by the flat, unchanged 

 endoderm lamella within the two rows of diverticula. Through this 

 the osculum will be formed. At the other end the basal endodermal 

 lamella closes the mouth of the gastrula. The distribution of the 

 diverticula and their definite modification into ciliated chambers are 

 described. The afferent pores arise either by fusion of the two layers 

 and consequent rupture, or more frequently the canal which feeds two 

 chambers seems to arise as an ectodermic invagination. Small isolated 

 chambers appear in the basal endodermal lamella. The mesoderm 

 increases greatly. 



(g) The formation of the osculum. — The ectoderm on the summit 

 of the hemisphere is more and more raised from the endoderm. An 



