So-called Mesectoderm in Petromyzon. 



473 



visceral arches, is the vascular. In the first formed vascular system, in 

 which the arterial and venous systems are not yet differentiated, we have 

 before us a vascular system of the Annelid type (Hatta (07), Keiser (14) ), 

 which consists of a dorsal and a ventral longitudinal vessel connected with each 

 other by vascular rings which are repeated in each body somite (Hatta (07), 

 (14a), Keiser (14) ). The anterior division of this ring vessel system is repre- 

 sented by the vascular visceral arches (Goette (90) ), and is followed by 

 Mayer's Quergefasse in the pronephric region, while in the region still further- 

 posterior the segmental character of this series of vessels is obscure, because 

 in this region there is no organ of segmental arrangement standing in direct 

 relation with the vessels. 



The origin of the ring vessels, whether entodermic (Goette (90)) or mesodermic 

 (Hatta (14a), Keiser (14) ), is by no means mesomeric, because the vascular 

 cells are derived, even in the strictly segmented branchial region, from an 

 organ which is not segmented in accordance with the mesodermic somites.* 



The pronephros is the only organ in which the vasomery, i.e. branchiomery, 

 is connected with the mesomery. But there is an incontestable fact which 

 shows that the segmental repetition of Mayer's vessels in the pronephric 

 region is not primary but secondary. The first pronephric artery, which is 

 evident in a larva of the 10th day, corresponds with the' 11th vasomere 

 (counting the premandibular vascular arch as the first vasomere), and the 

 first pronephric tubule with which the artery stands in relation, is the 

 product of the 7th mesodermic somitef (Hafcta (97), (00), (14a), (146)), 

 while this somite is no longer found over the first pronephric tubule, when 

 the tubule is cut off from it. 



There are two movements by which the segmental discordance between 

 the nephromeres and the myomeres is brought about, viz., the pronephros 

 is gradually pushed backwards by the outgrowth of the visceral pouches ; 

 the somites, i.e. myotomes, on the contrary, move forwards after their 

 formation, so that a few anterior of them are pushed over the preotic 

 section of the head to give rise to the capitis muscles. The ring vessels, 

 i.e. Mayer's vessels, occur in the visceral arches as well as in the 

 pronephros only when the visceral pouches are formed and the backward 

 movement of the pronephros has already been carried out. The vasomeres 

 are thus put secondarily in relation to the pronephros, although the latter 



* According to Goette, the vascular cells (in branchial region) are derived from the 

 enteric wall ; according to my results, which are confirmed by Keiser, the cells are 

 derived from the mesodermic visceral arches. 



t In my papers formerly published (97, 00) the first pronephric tubule is regarded as 

 the product of the fourth scleromyotome, which corresponds to the seventh, when the 

 three preotic somites are counted in. 



