HEAD-FOLD TO TWELVE SOMITES vt 
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 he behind the embryo and not to be a part 
of the embryo itself. The first somite hes 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 known 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 
