So-called Mesectoderm in Petromyzon. 



463 



grown downwards by active cell-multiplication (text-fig. 2). The growth is 

 produced not by the rearrangement of free cells cast off, but by the out- 

 growth of a continuous epithelium a single cell thick, which pushes its way 

 between the somatic layer of the lateral plates and the ectoderm until the 

 ventral edge of this epithelium reaches the mid-ventral line of the thyroid- 

 groove. The epithelium thus produced is what is called the mesectoderm. 



The downward growth of the mesectoderm can readily be traced step by 

 step. On the fifth day, where the growth of the layer begins, the ventro- 

 lateral edge, for instance, of the fifth scleromyotome* is produced a short 

 distance downwards and is wedge-shaped in cross-section. The cutis layer 

 of the scleromyotome passes over into the muscle plate round the apex of 

 the wedge. On the sixth day the mesectoderm in its anterior portion is so 

 broad that its lower margin is found as low as the thyroid groove, while 

 farther backwards the layer is narrowed so that it is represented in the 

 seventh scleromyotome by a short wedge-shaped process of the latter. It is 

 only in the course of the eighth day that the mesectoderm is fully estab- 

 lished in the posterior branchial region. 



The formation of the mesectoderm is, therefoi-e, commenced in the anterior 

 region and goes on backwards (text-fig. 3). In its early stages of formation 

 the mesectoderm is an epithelium composed of flattened cells ; but it 

 thickens gradually as its component cells assume a tall columnar character 

 which is, doubtless, brought about by their mutual pressure resulting from 

 repeated cell-multiplication within the layer. 



On a cross-section through a visceral arch six layers of epithelium are now 

 seen : the innermost is the entodermal pharynx wall ; the next outer feeble 

 layer represents the first rudiments of the vascular arch ; then follow the 

 splanchnic and somatic layers of the lateral plates representing the meso- 

 dermic visceral arch ; and between the latter layer and the outermost layer, 

 the ectoderm, intervenes an intensely stained epithelium of tall columnar 

 cells, which represent the mesectoderm. The mesectoderm is gradually 

 diminished in thickness from the level of its middle height downwards into 

 its sharp-edged lower margin. 



Although the mesectoderm is only a single cell thick in most parts, at the 

 dorsal edge, where it is in connection with the ventro-lateral edge of the 

 scleromyotome, it is divided into two layers which pass over into the cutis 

 layer and the muscle plate layer of the scleromyotome respectively. The 

 mesectoderm can, therefore, be regarded as a fold arising from the above- 



* The anterior three inesodermic somites found in front of the auditory vesicle are not 

 transformed into typical scleromyotomes ; nevertheless they are reckoned as such. The 

 fifth scleromyotome is accordingly the second postotic. 



