So-called Mesectoderm in Pelromyzon. 



465 



assume, analogous, and the two structures thus brought about are serially- 

 homologous. The difference consists in that the mesectoderm is destitute 

 of the muscle plate layer. Though to a very small extent, the mesectoderm is 

 divided into two layers, and there can be no objection to our regarding the 

 inner of these as homologous with the muscle plate. 



The causes of this modification in the branchial region are to be sought 

 in the changes in that region produced by the respiratory mechanism and 

 its skeletal framework. As will be stated later on, the mesectoderm is 

 converted to a great extent into the cartilaginous branchial bar, which is a 

 special skeletal arrangement for the respiratory mechanism. The remainder 

 of the layer supplies the elements for the subcutaneous tissue, while, in the 

 postbranchial region, the whole of the cutis layer is employed in the forma- 

 tion of this tissue. The demand for the formative elements of this tissue 

 has caused, as I believe, the cutis layer of the scleromyotomes in the branchial 

 region to be developed so vigorously as to call the mesectoderm into existence, 

 although the muscle plate in the branchial region is almost entirely sup- 

 pressed. The consequence of this suppression is that the somatic walls of 

 the branchial chamber are destitute of the segmental muscles, and have to fill 

 this deficiency by the so-called hypoglossal muscles which with their cutis 

 layers undergo an exceedingly modified mode of development,* as was 

 pointed out by Neal (97) and was confirmed by Koltzoff (02), j 1 and by 

 myself (14a, 146). 



The great modifications met with in the prootic section of the head cause 

 the mesoderm to be modified to a still greater extent than in the postotic 

 branchial region. Accordingly some peculiarities occur in the formation of 

 the mesectoderm in this region. 



In this section of the head there are formed three mesodermic somites, 

 the third being situated just in front of the auditory vesicle. Of these three 

 somites the hindermost shows a structure very similar to a postotic somite ; 

 the middle somite, under which the lateral plates^ develop into the enormous 

 mesodermic mandibular arch, is represented by a narrow epithelial fold of the 

 archenteric roof, which ascends along the lateral wall of the medullary canal 

 and strikes with its distal extremity against the trigeminal placode of the 



* The hypoglossal muscles are produced by the forward bending and shifting of the 

 ventral part of some postbranchial scleromyotomes. They are the only segmental 

 muscles in the somatic walls of the branchial chamber. 



t As Koltzoff (02) remarks, Goette (90) gives incorrectly four somites in front of the 

 auditory vesicle. 



I Koltzoff is the only author who gives a detailed account of his second somite. But 

 he failed to detect the free somite, which is very small, whilst he regards the dorsal part 

 of the colossal lateral plates as their somitic part. 



