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Part III. — Tivsnty -eighth Annual Report 



perfectly distinct up to the point where they joined the ventral aorta. They 

 differed from the five normal arches in this, viz., that they did not supply a 

 gill pouch, but running almost directly backwards, and then outwards, they 

 ramified among the muscles of the body wall. In specimen C the sixth arch 

 was found, but the vessels were smaller and more delicate than those in 

 specimen B ; moreover, they never actually entered the ventral aorta, but 

 ended blindly in connective tissue immediately posterior to the origin of the 

 fifth pair of afferent branchials. 



The only literature which threw any light upon this subject was the 

 following extract from Milne' Edwards "Legons." Speaking of the aortic arches 

 of fishes, he says : — " Les crosses aortiques se constituent successivement d' 

 " avant en arriere, et Ton en compte jusqu' a sept paires ; mais il est rare 

 " que tous ces vaisseaux aient une existence permanente, et, le plus 

 " ordinairement, les premiers formes s' atrophient et disparaissent avant que 

 " les derniers se soient bien constitues ; en tin d'autres fois quelques-uns de 

 " ceux-ci paraissent avorter, de sorte que, chez l'animal parfait, le nombre 

 " de ces arcs vasculaires ne depasse que rarement quatre ou cinq paires." 



In the case of the porbeagle, it would appear on the contrary that the 

 posterior, or last formed, arterial arch atrophies ; for it is this posterior arch 

 which is not found in the adult Selachian. 



From the evidence of these aortic arches, it appears highly probable 

 that during the younger stages of the development in Lamna comubica 

 there may be a kind of placental connection between mother and young. It 

 has been shown that the body-wall is continuous with the wall of the yolk- 

 sac ; further, that a supply of blood reaches the body-wall direct from the 

 heart. If, at an earlier stage than that under consideration, the wall of the 

 yolk-sac was actually in organic connection with the ridged inner wall of the 

 uterus (which is also well supplied with blood vessels), it is quite possible to 

 conceive that the venous blood brought by the sixth aortic arch to the wall 

 of the yolk-sac might be aerated by arterial blood in the uterine wall. 

 When the gills were formed this method of respiration would no longer be 

 necessary, and the accessory sixth aortic arch would atrophy. In specimen C 

 one might point out that such an occurrence is in the act of taking place. 

 The weak point in this idea is the fact that there is no trace on the wall of 

 the yolk-sac of any such organic connection as was postulated. 



The Uterus (Oviduct) of the Adult. 



The adult female which contained the two embryos A and B which have 

 been described above, measured about five feet in total length. Each oviduct 

 was 23 inches in total length. The oviducts were united by tough mesentery 

 throughout their length. The anterior portion of the oviduct was closed by 

 a dense network of tissue. The uterine portion was very much swollen in 

 order to contain the large embryos ; each oviduct contained one of the latter. 

 The uteri united at the posterior end, ran parallel with one another for two 

 inches, then opened by a common muscular tube about three inches in length. 



The uterine wall was of a leathery nature. The external surface was 

 smooth, the internal thrown into ridges and furrows. At the anterior end 

 of the uterus the inner wall showed the highest ridges ; these were in the 

 form of longitudinal folds, between which lay smaller elevations. The 

 remainder of the internal surface of the uterus was ridged in all directions, 

 the ridges growing less conspicuous towards the posterior end. No villi 

 comparable with those described by M'Intosh* for Zoarces viviparus were 

 observed. 



In transverse section (Plate IX., fig. 5) the uterine wall showed an outer 

 layer of compact cells, next a deep layer consisting of a series of muscle 

 fibres and nucleated cells, and finally a dentate layer composed of small 



* Annals and Mag. of Nat. Hid., June, 1885. 



