8 THE VOYAGE OF H.3[.S. CHALLENGER. 



curved ou itself tlian in the earlier stage, and the fronto-nasal process (ii-f.p.) almost 

 reaches the pericardium. 



The whole embryo is now clearly divisible into four regions, viz., the head, the 

 neck, the trunk, and the tail; this last is of nearly equal length to the others, but they are 

 also almost equal to each other in bulk. 



The proximate part of each limb lies adherent to the infero-lateral region of the body, 

 in a manner very similar to what is seen in the Osseous Fishes, where the moieties of the 

 shoulder- and hip-girdles are permanently arrested ; moreover, the latter are small, and 

 lie low down at the ventral edge of the body-wall in those types. 



The shoulder-plate adheres like a Limpet to the side of the bulbous chest, and it 

 is in reality the outer layer of the imperfect somatopleure, growing down over that 

 huge diverticulum of the splanchnoidleure — the pericardium — which encloses the 

 heart. 



The hind Jiipper is still a mere discoid expansion of the growing limb ; the fore 

 flipper is growing into angles, the hinder of which will soon become the elongated part 

 of the inanus : it lies outside the lower edge of the incijDient carapace. 



The supero-lateral segments are still more sharply divided into the crowded series of 

 the neck, and the widening series of the trunk ; below the latter a very definite marking 

 off of the lower edge has taken place ; these smaller areas will contain the intercalary 

 dermal plates. The deep sulcus between the marginal row of cutaneous folds and the 

 ingrowing abdominal part of the body-wall is ultimately bevelled down, but it shows well 

 that the plastron is not formed in a structure which was flat originally, but trough- 

 shaped. 



Of course, in a body so built in as that of the Tortoise, the " muscle-plates " of the 

 dorsal region are to a large extent aborted ; and the mass of the mesoblast has to be 

 converted into the connective tissues into which the costal ectostoses run. 



One pair of muscle-plates, at any rate, behind the ear, will belong to the occipital 

 arch, as they are largely inserted into it ; this gives us the hinder cranial limit. It is seen 

 behind the lageniform ear-sac (an.) ; this lies directly over the fourth post-oral cleft, now. 

 rapidly closing up. 



The intervening folds are being packed close together, and are at present narrow 

 arched bands of tissue ; only the first and second of these will enclose a permanent 

 cavity, which, however, will be completely sealed up externally. 



Much of the metamorphosis taking place within these parts is now visible externally, 

 the outward form is being remoulded most effectually by the inward growth-processes. 



The elevations now seen on each side of the hind-brain are as follows : — (a) The 

 auditory sac (au.) as an obliquely-placed pouch, with its neck (primary involution) 

 looking upwards and forwards. 



(h) Four elevations below this, namely, in front, the crescentic upper region of the 



