So-called Mesectoderm in Petromyzon. 



469 



mesectoderm. The proximal half of the last-named epithelium is thickened 

 so as to be. raised inwards into a ridge, which shows on cross-sections a 

 pyramidal outline and represents the first rudiment of the cartilaginous 

 visceral arch. This stage of differentiation is represented in text-fig. 18 by 

 Schalk, which is correct, except the connection of the mesectoderm with the- 

 ectoderm. 



The cells forming the rudiment of the cartilage acquire a radial arrange- 

 ment (text-fig. 1, A) and are soon constricted off from the remainder of the 

 layer ; this stage is followed by a stage in which they are wedged in between 

 one another, so as to form one row (B). On cross-sections through a larva of 

 the 14th clay this rudiment of the cartilage looks like a bar consisting of 

 piled-up discs, in which three sections are distinguishable (C) : a dorsal and 

 a ventral section curved outwards and the middle section bowed inwards. 

 While the ventral section touches with its distal extremity the lateral 

 division of the vena jugularis impar, the proximal end of the dorsal section 

 lies in the corner between the dorsal aorta and the chorda and under the 

 anterior cardinal vein. In the course of the 15th day, the aorta together 

 with the roof of the pharynx is separated from the chorda by enormous 

 development of the reticular subchordal connective tissue. Accordingly the 

 dorsal end of the rudiment of cartilage is also brought downwards, so as to be 

 forced into the corner between the aorta and the roof of the pharynx which 

 has been pressed down. It is interesting that the band of connective tissue 

 which before and after this change connects the rudiment of cartilage with 

 the chorda is drawn out into a string stretched between both the structures. 



In sharp contrast to other visceral arches, the hyoid arch does not undergo 

 this dislocation of the cartilage bar, which is, on the contrary, shifted by 

 stages a little upwards, and the cardinal vein passes into the mandibular vein 

 under the cartilage bar. This peculiar feature is the first step towards the 

 fulfilment of the function which the hyoid arch has secondarily acquired ; it 

 enters into the formation of the primordial skull, leaving the service of 

 respiration. 



The differentiation of the mesectoderm into the visceral arch takes place 

 at first in the visceral arch behind the hyoid arch and proceeds backwards to 

 the following arches, which undergo the same process one after another. For 

 a long while the mesectoderm in the hyoid arch is not cut off from that in the 

 mandibular arch. And it delays its differentiation into the rudiment of the 

 cartilaginous hyoid arch, which is, however, obvious before the same process 

 commences its work in the hindmost visceral arch. 



The rudiment of the cartilaginous visceral arch shifts inwards, when it is 

 detached from the remainder of the mesectoderm, and presses and separates 



VOL. LXXXV1II. — b. 2 p 



