Embryology of the River- Lamprey. 41 



Then, too, preparations occur in whicli the lateral plates have 

 not yet completely surrounded the lower surface of the 

 intestine ; their two layers lie close to^yether and more 

 towards the periphery, and yet in the middle of the body- 

 cavity we find an endothelial pellicle belonging to the future 

 endocardium. In spite of all these appearances I consider 

 that I am bound to adhere to the opinion that the endocardium 

 has separated off from the lateral plates on the inner surface 

 precisely in the same manner as on the outer. I find myself 

 compelled to do tliis by the arrangements presented by quite 

 young stages of the embryos, measuring about 2*5 millim. in 

 length. Tiie endocardium is here visible in all sections taken 

 in the cardiac region, but always in the closest connexion with 

 the niyocardiuni. Both membranes appear thick, and are 

 richly provided with yolk-granules. 



Before closing this division of my paper, I must not pass 

 over in silence the statements of A. Goette, since they are at 

 variance with my observations. According to tliis author, the 

 endocardium is formed from endoderm. Since it is difficult to 

 suppose that so capable an investigator as Goette should have 

 been mistaken, the material investigated must have been of 

 such a kind as to admit of what was seen being interpreted in 

 a different fashion. Goette's view might be supported by the 

 circumstance that in the neighbourhood of the rudiment of the 

 heart the oesopliagus is rather strongly compressed at the sides, 

 and almost touches the ectoderm with its under surface. When 

 the formation of the heart has taken place, the lumen of the 

 oesophagus has diminished by one half. Isolated cells are 

 then not infrequently to be found in the upper angle formed 

 by the contact of the intestinal fibrillar plates. A constriction 

 of the oesophagus, as has sometimes been described, or a 

 splitting-off of a series of cells from its under surface for the 

 benefit of the endocardium, I have never seen, and I therefore 

 adhere to the assumption which I have stated above. 



The Endoblast. 



The branchial cavity arises by the separation of the cells of 

 the yolk at the anterior end of the embryo, in the manner which 

 we have had occasion to observe in the formation of the intestine. 

 The great thick head, consisting of a compact mass of cells, com- 

 mences to recede somewhat from the yolk^ and becomes smaller 

 and narrower. Between it and the yolk the anterior portion 

 of the body of the embryo becomes continually more distinct 

 and longer. The internal changes proceed hand in hand with 

 the external ones. The cavity which has been formed, which 

 was at first more spacious at the anterior end, gradually also 



