660 R. C. PUNNETT. 
Lastly there is a point in connection with the branchiae which calls for short notice. 
It has been assumed that the main function subserved by these structures is that of 
respiration, and in the figures given by most authors the blood vessels form a prominent 
feature. Nevertheless the course of the branchial circulation is a point which has never 
been satisfactorily cleared up (cf. Delage and Herouard, ’98, p. 41), and in looking through 
numerous series of sections through the branchial region I have been struck by the infrequency 
with which one finds any blood in these structures. On the other hand the sub-epidermal 
vascular plexus is exceedingly well-developed everywhere and it is difficult to avoid the 
conclusion that respiration is mainly, if not entirely, carried out by the large skin area, whilst 
the main function of the gills is to act as a sieve for straiming off the water from the 
mixture of sand and water continually swallowed by the animal. 
THE FORMATION OF THE GONADS. 
The origin of the gonads is a subject on which widely diverse views have been upheld 
by different observers. Bateson (?86) attributed to them an ectodermal origin, Spengel (795) 
regards them as being developed in a blood space, whilst Morgan (?94, p. 60) holds that they 
are formed from the mesodermal elements of the coelom. The question is one of considerable 
difficulty and none of the above observers has made out a very strong case in support of 
his view. Bateson bases his belief on the early connection apparent between the gonad and 
the ectoderm (86, Pl. XXXII. fig. 110). Spengel considers this connection to be due to 
the precocious formation of a duct and claims to have found sex cells at a very early 
stage of development inside the blood vessels (95, Pl. II. figs. 27 and 28). He finds 
additional support for his view in the fact that in a small specimen’ of Pt. flava, var. 
laysanica, the “Anlage” of the gonad first appears between the two lamellae of the lateral 
septum (’08, p. 302). The case which he has made out is far from convincing since it 
is by no means certain that the small cells which he regards as primitive sexual cells are 
of that nature, whilst on the other hand there are good grounds for looking upon the 
lateral septum as composed of four rather than of two lamellae (see below, p. 661). Lastly 
Morgan’s contention as to the mesodermal origin of the gonads is not borne out by his 
figures. In the earliest stage figured by him (94, Pl. VI. fig. 79) the gonad is obviously 
im continuity with the ectoderm, a connection which, to judge by his figures, is very soon 
lost, though doubtless re-established later on the formation of the genital ducts. The question 
of the ectodermal origin of the gonads turns largely upon the interpretation of the ecto- 
dermal connection found at different stages of their development. In other words, has the 
gonad an ectodermal connection at two different stages of its growth, the earlier representing 
the ingrowth of ectoderm to form the gonad, whilst the latter represents the functional 
duct? From the following observations on Pt. flava I am inclined to believe that such is 
the case. As the gonads appear to develop earlier in some varieties than in others I have 
considered them apart with the following results. 
(1) Var. laccadivensis. The youngest stages of the gonads were met with in a very 
small specimen about 15 mm. long. ‘The young gonads are in many places in connection 
with the ectoderm (Pl. XL. fig. 36). In the region of the genital pleurae this connection 
1 Judging from the regenerated material at my disposal in doubt in considering this small specimen of Spengel’s to be a 
the case of Pt. flava, var. laccadivensis, I have very little regenerated one. 
