784 SUMMARY OP CURRENT RESEARCHES RELATING TO 



wards. Anteriorly and posteriorly, however, a slightly different mode 

 of format ion of the mesoderm obtains. At the two ends the longi- 

 tudinal grooves aro absent, and tlic ventral plate here becomes 

 thickened and stratified along the middle line ; then the superficial 

 layer (ectoderm) becomes separated from the deeper layer (mesoderm). 

 The latter, which is continuous with the central portion of mesoderm, 

 gradually grows round so as to occupy the space which has appeared 

 in the vitellus ; here it is gradually folded on to the dorsal surface, 

 the sides progressing more rapidly than the median portion. As it 

 grows backwards, the thickness diminishes, and at the edge consists 

 only of a single layer of cells, which the author regards as the most 

 anterior portion of endoderra ; which is thus derived from tho 

 mesoderm, and pushes its way between the yolk and the ectoderm. 

 It is difficult to say where the mesoderm commences to become 

 endoderm. The anterior and posterior portions of endoderra 

 approach and ultimately meet together. 



Meanwhile, the embryo is becoming shorter, as the ventral plate 

 curves more and moro to the dorsal surface. 



The derivation of the endoderm from mesoderm is contrary to the 

 opinion of Balfour, Dohrn, &c, who regarded it as formed from 

 certain cells remaining in the yolk. Prof. Grassi denies that any such 

 " secondary segmentation " of the yolk takes place, although proto- 

 plasm with nuclei in it forms a syncytium in the yolk, and in a later 

 stage this gives rise to the granular material which fills the mesen- 

 teron. Moreover, these cells are never in rows, so as to give rise to 

 any membrane. 



d. The nervous system. — The cerebral ganglia are formed as 

 two thickenings of the ectoderm, which do not become united till 

 late in development. The fossa observed by Hatschek in lepidopter- 

 ous embryos, appears after the ganglia are detached from the ectoderm. 

 The ventral chain arises as two longitudinal swellings, which are at 

 first quite separate ; the whole thickness of the ectoderm is concerned 

 in the formation, which sinks, and the ectoderm grows over the chain. 

 The transverse commissures appear, from ectoderm, later on. 



e. Tracheal system. — The ten pairs of stigmata are the first to 

 appear, before the limbs show themselves, and after the amnion is a 

 complete sac. The anterior ones appear first ; and they are all formed 

 on the latero-ventral line, before there are any signs of segmentation. 

 The inpushed ectoderm, which gives rise to the stigma, grows in, 

 divides into an anterior and a posterior branch, which push their way 

 in between ectoderm and mesoderm. The lateral tracheal trunk is 

 united to its fellow of the other side above the oesophagus and above 

 the rectum. The " spiral filament " appears later on. Up to the 

 time of hatching the tracheae are filled with the amniotic liquid. 



/. Alimentary tract. — The stomodaeum makes its appearance 

 simultaneously with the tracheae ; the proctodeum a little later. 

 The former has a pit just behind the procephalic prominence. This 

 pit deepens, as the yolk recedes, and enlarges at its inner end to form 

 the gizzard. In the posterior region of the dorsal surface, two 

 pairs of little pits become formed, which will give rise to the four 



