ON THE EMBRYOLOGY OF THE TELEOSTS. 209 



the epiblast, and he also ('81) regards the space into which the somatopleure thus forces 

 itself as the remains of the segmentation cavity. So far as our eggs and our observations 

 go there is not the slightest evidence in favor of either view, for the epiblast is everywhere 

 in close connection with the mesoblast and nothing could be seen in the envelopes of the 

 yolk which could in any way be interpreted as remains or derivations of the segmentation 

 cavity, a cavity of which all traces are lo^t at an early stage in the development. In our 

 eggs also there was a veritable splitting of the mesoblast as will be seen by an inspection 

 of fig. 36. 1 



From this point on my observations are exceedingly fragmentary and the account would 

 best be confined to little more than the remaining figures, a course of procedure which 

 will serve to connect the earlv stages with those of which Mr. Asrassiz treats. 



In a side view, at first but two of the prominences of the brain are seen (fig. 33) but 

 scon the third, the mid-brain, makes itself visible as shown in fig. 34, and at the latter 

 stage we first find the traces of the pericardium and the heart (p. fig. 34). Of the origin 

 of these portions of the anatomy I can say but little. It seems almost impossible to cor- 

 relate the steps of their development with those found in other vertebrates. At first 

 there appears a mass of mesoblastic tissue arising almost beneath the hind brain and pro- 

 jecting into what has previously been regarded as the cavity produced by the splitting of 

 the mesoblast (and therefore corresponding to the pleuroperitoneal cavity of other verte- 

 brates) but which is regarded by Ryder as the segmentation cavity. (We shall return to 

 this cavity further on). This mass of cells grows downward and when it comes in contact 

 with the lower layer of cells (either hypoblastic or sphlanclmopleuric) a lumen appears, 

 the primitive heart. At first this is a simple tube and indeed for a considerable time 

 retains that character as shown in figs. 40 and 50. When the heart first begins to pul- 

 sate the vibrations are very slow and frequently it ceases beating for some time and then 

 begins again. Coincident with the first pulsations, which appear at a little later stage than 

 that represented in fig. 43, the first motions of the embryo are seen, and consist of slight 

 tremors of the whole body. At first the contractions of the heart produce no currents of 

 blood nor in fact are any corpuscles to be seen. In the eggs of the dinner it is extremely 

 difficult to trace the development of the circulatory system after this time, only slight and 

 unsatisfactory views of portions being visible, while only once was I able to see anything 

 whatever of the blood vessels of the yolk sac. Of the formation of the corpuscles nothing 

 was seen nor were they visible until a comparatively late stage. 



We have seen in preceding pages the method of formation of the hypoblast of the ali- 

 mentary tract by an invagination to which are possibly added cells from the intermediary 

 layer. A portion of the hypoblast eventually forms a solid cord extending along beneath the 

 body but of the exact method we are not certain. This cord gradually grows forward and at 

 intervals a lumen appears, as shown in fig. 33 me., the future cavity of the canal ; this is shown 

 again at a later stage in figure 42, which represents the hinder end of the mesenteron of a 

 fish about as far developed as shown in fig. 43. Though the exact process of the closure of 



1 The investigations of 1 882 which embraced these points some modification, the changes required are far less than those 

 lead me to regard Mr. Ryder's conclusions more favorably, implied in the above paragraph which is based upon some 

 and though I am not ready to accept them wholly without erroneous interpretations. 



MEMOIRS BOST. SOC. NAT. HIST. VOL. III. 27 



