OF TJ113 TELEOSTEAN FISII SEBASTBS MAUINUS. 503 



position, but has shifted further forward than in the 4-5 mm. 

 stage. The auditory vesicles are fairly well advanced and can be 

 seen clearly in whole mounts. Large pigment cells are to be 

 seen over the posterior part of the gut, and there is a row along 

 the ventral side of the tail. The 25 mm. specimens are of a post- 

 larval stage and exhibit roughly the form of the adult. 



Reconstructions were iii each case made from series of transverse 

 sections cut at 10 /x. These were examined with the aid of a 

 squared eyepiece micrometer, and lateral and dorsal views were 

 plotted directly on to squared paper. The details of this method 

 are described in a recent paper by Wells (1922) on the chondro- 

 cranium of Clupea. As it is necessary in this method to have a 



• 



Text-figure 2. 

 N --' Aud.c. 



"■'---- Tr. 



Stage 1. Lateral view of clioiulrocraiiiuin. 



base line from which all measurements are made it is usual to 

 assume that some element in the head, generally the notochord 

 or some piece of cartilage, is straight. I have been able to avoid 

 this assumption by plotting an accurate outline of the head 

 before sectioning it, and then taking all measurements from the 

 outline. Other methods of reconstruction are described by Norman 

 (1923) in a paper dealing specially with the subject. 



3. The Chondrocranium 



a. Stage 1 . The unhatched embryo. 



This is the earliest stage it was found piofitable to investigate 

 (text-fig. 2). The head skeleton is in an extremely elementary 

 condition, and is represented principally by tracts of procartilage 

 and connective-tissue cells which are becoming concentrated into 

 the positions in which cartilage is subsecjuently to be laid down. 

 It is always a mstttor of soino difficulty to draw a definite line 

 between procartilage and cartilage proper. The trabeculjB at 

 this stage appear to be in an intermediate condition. The outline 

 is quite definite, but the cells have not become thoroughly welded 

 together to form a solid matrix. The trabeculoe are very short 



