592 SUMMARY OF CURRENT RESEARCHES RELATING TO 



becomes differentiated into a two-layered endo-mesodcrm. The 

 boundary between tbo external layer next the ectoderm and the 

 internal layer next the yolk represents the compressed lumen of the 

 primitive tube, i. e. the primitive enteric cavity of the modified gas- 

 trulation. The histological differentiation of the two layers is dis- 

 cussed in detail, and Dr. Heider shows how from the external, the 

 somatic mesoderm arises, and from the inner, the endoderm and mus- 

 cular lining of the alimentary canal, (e) The boundary between the 

 outer and inner layer enlarges on either side to form clefts which 

 increase in width and form the primitive cavities of the segments. 

 The lateral portion of the inner layer which thus bounds these clefts 

 forms, for the most part, the muscular layer of the mid-gut, while the 

 median portion, becoming histologically differentiated, forms the 

 epithelial layer of the same, becomes in fact the final endoderm layer. 

 (/) The body-cavity, formed independently of the primitive segment 

 cavities, appears as a cavity between the yolk and the somatic meso- 

 derm layer. The details of these processes, and of others, such as 

 the growth of the ectoderm round the yolk, are noted. It is interest- 

 ing to note that just as the anterior and posterior defining of the 

 furrow-like invagination originated independently, so the endoderm 

 layer is formed in two separate anterior and posterior portions which 

 subsequently grow together across the intervening gap. 



Dr. Heider describes (g) the condition of the yolk with persisting 

 cellular bodies, and with cells which wander inwards from the endo- 

 mesoderm, and emphatically denies the frequently asserted formation 

 of part of the internal layer from the apposition of nuclei from the 

 yolk. The degeneration of the latter, and its absorption by pseudo- 

 podia-like processes from the endoderm cells is noted. While agree- 

 ing with Kowalevsky, as against numerous other investigators whose 

 views he criticizes, as to the origin of the endoderm from the invagi- 

 nated lower layer, Dr. Heider differs from him on two chief points — 

 the origin of the primitive segment cavities and the mode in which 

 the endoderm is separated off. For while Kowalevsky derives the 

 former from internal and inferior foldings of the endo-mesoderm, 

 and represents the folded portion as consisting of both endo- and 

 mesoderm afterwards separated, Dr. Heider has described the origin 

 of the cavities from a lateral split between the two layers of the 

 invaginated compressed tube, and derives the endoderm from the 

 median portion between them. This median portion afterwards 

 grows in between the splanchnic mesoderm and the surface of the 

 yolk, which then becomes enclosed dorsally and ventrally by the 

 simple extensions of the two layers. 



As subsidiary results of his research, Dr. Heider notes (7») that 

 not only on the first abdominal segment, but on all the others rudi- 

 mentary appendages can be observed ; (i) that the transverse com- 

 missure of the ganglionic chain originates by the invagination between 

 the lateral strands of a median portion retaining an intersegmental 

 connection with the ectoderm, that the oesophageal commissure is 

 formed from the anterior portion of the lateral cords without 

 assistance from the mandibular ganglion, that the apical plates are 



