502 



MR. N. A. MACKINTOSH ON THE CUONDROCRANIUM 



2. Material and Methob. 



Some difficulty in obtaining all the desirable material has 

 rendered the series of developmental stages a little incomplete, 

 for I was unable to procure any specimens between the 5*5 mm. 

 and 25 mm. stages. 'I^his leaves rather a large gup during which 

 a consideral)le development of cartilage takes place, it is not 

 difficult, hoAvever, to reconstruct the more important processes 

 which occur during this period. 



I had at my disposal large quantities of specimens from the early 

 segmentation stages up to the 55 mm. stage, and three or four 

 specimens ranging between 25 and 30 mms. The abundance 

 early stages compensates to some extent for the lack of some of 

 of the later stages. 



Text-figure 1. 



Stage 3. External features. 



It has been found most convenient to divide the following 

 account into four parts : — 



Stage! . The unhatched embryo. 

 ,, 2. The 4-5 mm. larva. 

 „ 3. The 5'5 mm. larva. 

 ,, 4. The 25 mm. post-larval stage. 



Stages 2 and 3 are close together, but the dilFcrencos betweoii 

 them are sufficient to justify their separate consideration. The 

 specimens for stages 1-3 were supplied from Lowestoft and those 

 for the 25 mm. stage Avere supplied by the Board of fisheries for 

 Scotland at Aberdeen. Fixation was in every case in formalin. 

 Consequently it was not easy to follow much of the histological 

 details of the formation of cartilage, though preservation was 

 otherwise satisfactory. The principal stains used were thionin 

 and orange G, Mallory's triple stain and picronigrosin. For the 

 first stage specimens were removed from the egg capsule for 

 sectioning. The diameter of the curled up embryo is at this 

 stage about 1 mm. The eyes can already be seen with the naked 

 eye and serve as a useful guide for orientation. The 4-5 mm. larva 

 has just escaped from the egg capsule and still retains a large 

 yolk sac. In most respects it closely resembles the 5-5 mm. larva. 

 In the latter (text-fig. 1) the yolk sac is considerably reduced 

 though it is still a fairly substantial body. The principal features 

 of this larva are the vei-y long tail, the large eyes, and the distinct 

 cranial flexure. The mouth is still in a more or less ventral 



