No. I.] GAR-PIKE AND STURGEON. 27 



as his observations have extended, the characters of this form 

 show a close agreement with those which Ruckert has described 

 in Selachians ; the present writer finds, for example, that the 

 vascular mesenchyme cells become early associated in loose 

 clusters in front of the head region of the embryo, and these 

 he has traced assuming definite shape as a blood tube ; he 

 notes that the early blood cells resemble closely the cells of 

 the early mesenchyme and that these seem to have their origin 

 in the anteriorly extending yolk-margin of the vascular cavity. 



The early embryo has clearly attained its outward form in the 

 stage figured in PI. I, Fig. 20, in which seven primitive seg- 

 ments are present. The outline of its trunk has become con- 

 stricted off from the yolk ; the eminences both of head and tail 

 are rounded and prominent, the mid-region of the trunk is well 

 raised above the yolk, and its parietal lamellae are now sharply 

 marked as a groove on either side of the embryo. In the dark 

 lateral margin of each parietal layer the pronephric duct has 

 become established. The advancing structures of this stage 

 which are to be briefly noted are (i) the brain vesicles, (2) 

 optic capsules, (3) the establishment of a lumen in the anterior 

 nerve axis and (4) pronephric duct. 



The changes of the anterior part of the central nervous sys- 

 tem of a stage slightly later than this have been described by 

 Balfour and Parker. The first appearance of a lumen in the 

 embryonic brain is to be found in the present stage. The 

 faintly marked axis of the head region denotes the wedge-like 

 insinking of the brain region, and its irregular margin is at 

 present the only outward indication of the vesicular enlarge- 

 ments. Sections show, however, that the anlage of the optic 

 vesicles exists in a grouping of cells on either side of the axis 

 at the broadest part of the cephalic plate ; and further that the 

 lumen which is found within the central axis first occurs in the 

 mid-region of the brain, arising, as Balfour and Parker be- 

 lieved, from the disassociation of cells : that this cavity, how- 

 ever, occurs before that of the optic vesicles is demonstrable, 

 and even in a stage 24 hours older than this. The English ob- 

 servers have figured a similar condition {Ref. 5, PI. XXII, Fig. 

 26), although they state the lumen as there probably occurring, 



