CH. VI] FORMATION OF THE GERM-LAYERS 69 



THE ORIGIN OF THE MESODERM 



3) It is difficult to give an account of the method of de- 

 velopment of the mesoderm, because there are almost as many 

 different descriptions of the process as authors who have de- 

 scribed it. I have without hesitation set aside those accounts 

 where the author has transparently sought to find his precon- 

 ceived theories demonstrated in his drawings of the sections of 

 the embryo. In the second place, several of the more recent 

 accounts have started out, I think, with a false conception of 

 the position of the embryo on the egg and its method of for- 

 mation, hence in these accounts the method of the formation of 

 the mesoderm is likely to be erroneously described, although 

 in several cases the actual drawings of the sections have been, 

 I believe, accurately made. I have followed as far as possible 

 those interpretations that are in conformity with the experi- 

 mental results relating to the growth of the embryo. Certain 

 abnormal embryos, to be described later (Chapter VII), that 

 first appear as a ring around the egg throw, I think, also much 

 light on the subject. 



The cells that are to form the mesodermal layer are present 

 at the time when the dorsal lip of the blastopore has first 

 appeared, and even just prior to that time. The innermost 

 of those cells forming the ring around the egg are the cells 

 that become the mesoderm (Fig. 19, B). These cells are 

 carried up to the median dorsal line of the embryo by the 

 closure of the blastopore (Fig. 24, A, B, C). They will then 

 be found forming a layer or sheet of cells (Fig. 25, B) that 

 separates itself on the outer side from the thick layer of small 

 ectodermal cells (that has been simultaneously lifted up) and 

 that is separated on the inner surface, but not very sharply if 

 at all, from the dorsal and dorso-lateral walls of the archen- 

 teron. A continuous sheet of tissue is formed in this way 

 over the dorsal surface stretching across the middle line. 

 According to some accounts, the fusion of this mesoblastic 

 sheet with the endoderm is much closer in the mid-dorsal line 

 than on each side. We may, however, think of the mesoder- 

 mal layer and endodermal layer as coming up together to the 

 median line from the sides, so that we are to think of the 



