Emhryology of the River -Lam prey. 39 



hypoblast. We may consider that the yolk-spheres during 

 the period which is now being described are on the average 

 from nine to ten times larger than the granules alluded to : 

 the size of many of them, however, is greater than this. 



Further and more important changes, which already stand 

 in direct connexion with the formation of the heart, are to be 

 noted in embryos of from 200 to 220 hours, when they have 

 attained a length of from 2 to 3 millim. Beneath the 

 branchial cavity an expanded oval region is formed, just as 

 if the embryo was somewhat swollen at this point. The 

 longitudinal sections sliow that a wide cleft has here appeared 

 in the lateral plates on the right and left : the outer layer 

 applies itself to the skin, the inner one to the intestine. We 

 then have on both sides of the intestine two oval vesicles, the 

 greatest diameter of which lies in the vertical direction. The 

 formations just described vividly remind us of the pleural sacs 

 of the higher Vertebrates before the appearance of the lungs. 

 In certain sections they appear quite empty ; in others, taken 

 higher up, we find the rudiments of the segmental organs, 

 that is of the primitive kidneys. 



In order to obtain a distinct representation of the develop- 

 ment of the heart, we must have recourse to transverse 

 sections. If we examine one of these from the cardiac region, 

 we observe round the oesophagus a wide free space, which has 

 been formed by the divergence of the two lateral plates : this 

 is tlie body-cavity. We employ this terra in order to apply a 

 general idea to this cavity. The intestinal fibrillar plate 

 (" Darmfaserplatte "), or splanchnic mesoblast, can be traced 

 particularly well in stained sections. We observe a fold of it 

 on the right, as also on the left side, which passes round the 

 segmental organs and the upper half of the oesophagus. When 

 the two layers meet beneath the closed oesophagus they assume 

 a downward direction, and finally pass into the somatic meso- 

 blast, which attaches itself to the ectoderm. At the spot at 

 which tlie right and left fibrillar plates have approached one 

 another — namely, where the two portions lying between the 

 splanchnic and somatic mesoblast are opposite to one another 

 — there is at first a narrow, and subsequently a much broader 

 cleft, which extends from above downwards. This inter- 

 mediate space is the cardiac cavity. The above-mentioned 

 portions of the intestinal fibrillar plates had fused together at 

 their upper and lower margins and become a closed tube, 

 which now projects freely into the body-cavity and is attached 

 to the intestine by a short band. Before the fusion of the 

 median portion of the intestinal fibrillar plates to form the 

 cardiac walls, there could already be observed on their inner 



