HEAD-FOLD TO TWELVE SOMITES 111 



and the whole tissue of much looser texture in the more anterior 

 portions of the embryo. 



The paraxial mesoblast increases rapidly in thickness and 

 thus becomes clearly distinguishable from the lateral plate. 

 Shortly after the formation of the head-fold a transverse split 

 appears in the paraxial mesoblast a short distance in front of the 

 anterior end of the primitive streak (Fig. 48). This is soon fol- 

 lowed by a second split, a very short distance behind the first, 

 and thus a complete mesoblastic somite is established. The split- 

 ting is accomplished rather by segregation of the cells than by 

 an actual folding. The mesoblast cells immediately in front of 

 the first split aggregate so as to form a somite continuous 

 anteriorly with the mesoblast of the head and thus lacking an 

 anterior boundary; this is the first somite, and the one formed 

 between the first two splits in the mesoblast is the second. 



The first somite established is first, not only in point of time, 

 but also in position, all the remainder forming in succession behind 

 this (cf. Figs. 48, 50, 51, 59, 61, etc.). As this is a point of con- 

 siderable importance for understanding the topography of the 

 embryo, and as previous text-books have a different account of 

 it, it is worth while to give the evidence for this position in some 

 detail. It has been believed up to a very recent time that from 

 two to four somites were formed in front of the first one. This 

 belief was due very largely to a misconception of the nature of 

 the primitive streak, which was believed by some to be extra- 

 embryonic, that is to lie behind the embryo and not to be a part 

 of the embryo itself. The first somite lies so near to the anterior 

 end of the primitive streak that it was difficult to believe that 

 room could be made by growth between it and the primitive 

 streak with sufficient rapidity to accommodate the rapidly form- 

 ing somites. In the entire absence of differentiated organs it was 

 impossible to find landmarks by which to distinguish the first 

 somite among the first five or six; hence it was natural to suppose 

 that a certain number of somites arose in front of the first, espe- 

 cially as it was not know^n how much of the anterior portion of 

 the embryonic axis represented the head. However, in the 

 absence of natural landmarks identifying the first somite formed, 

 it is quite possible to create artificial ones, and in this way to 

 identify it in later stages. This has been done by one of my 

 students, Miss Marion Hubbard, in the following manner: In the 



