612 H. H. SWINNERTON. 



allows the usual condition^ that of the other the unusual. 

 In the other seventli-day embryos there is no discontinuity 

 between the chordal and prechordal portions of the cranium, 

 tliough the two regions can be easily recognised by their 

 difference in calibre. 



iStage II. — As already described, tlie intercranial uoto- 

 cliord is no longer strongly flexed, but is now almost in a 

 line with that of tlie trunk, and has, indeed, undergone a 

 straightening out (fig. 58, ch.) in the process of hatching. 



Comparison of a dorsal view of the craniuin (fig. 2) at this 

 stage with that of the previous stage (fig. 1) gives, at first 

 sight, the impression that the notochord {ch.) has undergone 

 a reduction. This, however, is only an appearance due to 

 the more rapid growth of the surrounding parts. By com- 

 paring median longitudinal sections (figs. 57, 58) of the 

 respective stages, it becomes evident that the notochord {ch.) 

 not only does not decrease, but undergoes an actual increase 

 in absolute length. 



The parachordal tracts (fig. 2, p. ch.), which are no longer 

 divisible histologically into mesotic and occipital sections, 

 have grown considerably in all dimensions, quite indepen- 

 dently of the notochord {ch.), and their anterior ends (tr/) 

 are now situated far in front of the end of this. The space 

 (p. ch/) situated between them, which received the freely 

 projecting half of the notochord in the first stage, has been 

 carried forward, and now receives only its extremity. This 

 interparachordal space, as in the salmon (Parker, 73, 

 p. 129), is continuous in front with the pituitary fossa {pt.f.). 

 The anterior lateral process of the parachordal has also 

 undergone a similar forward shifting, by means of which it 

 now lies in a level with the extremity of the notochord. The 

 oblique ridge, in the mesotic region, is less strongly marked, 

 and owing to the increased width of this it no longer extends 

 to the notochord, but runs along the middle line of the 

 parachordal plate to the occipital section. The otic portion 

 of the plate, Avhich may be regarded as part of the auditory 

 capsule, occupies a lateral and not a postero-lateral position 



