In Teleostei the pronephros has precisely the same characters, except that the cavity in which the glomerulus is placed is without a peritoneal canal.
The questions which naturally arise in connection with the pronephros are: (1) what is the origin of the above cavity with its glomerulus; and (2) what is the meaning of the ciliated canal connecting this cavity with the peritoneal cavity?
We have not from our researches been able to answer the first of these questions. In Teleostei, however, the origin of this cavity has been studied by Rosenberg[541] and Götte[542]. According to the account of the latter, which we have not ourselves confirmed but which has usually been accepted, the front end of the segmental duct, instead of becoming folded off from the body-cavity, becomes included in a kind of diverticulum of the body-cavity, which only communicates with the remainder of the body-cavity by a narrow opening. On the inner wall of this diverticulum a projection is formed which becomes a glomerulus. At this stage in the development of the pronephros we have essentially the same parts as in the fully formed pronephros of Lepidosteus, the only difference being that the passage connecting the diverticulum containing the glomerulus with the remainder of the body-cavity is short in Teleostei, and in Lepidosteus forms a longish ciliated canal. In Teleostei the opening into the body-cavity becomes soon closed. If the above comparison is justified, and if the development of these parts in Lepidosteus takes place as it is described as doing in Teleostei, there can, we think, be no doubt that the ciliated canal of Lepidosteus, which connects the pronephric cavity with the body-cavity, is a persisting communication between this cavity and the body-cavity; and that Lepidosteus presents in this respect a more primitive type of pronephros than Teleostei.
It may be noted that in Lepidosteus the whole pronephros has exactly the character of a single segmental tube of the mesonephros. The pronephric cavity with its glomerulus is identical in structure with a malpighian body. The ciliated canal is similar in its relations to the peritoneal canal of such a segmental tube, and the coiled portion of the pronephros resembles the secreting part of the ordinary segmental tube. This comparison is no doubt an indication that the pronephros is physiologically very similar to the mesonephros, and so far justifies Sedgwick's[543] comparison between the two, but it does not appear to us to justify the morphological conclusions at which he has arrived, or to necessitate any modification in the views on this subject expressed by one of us[544].
The genital ducts of Ganoids and Teleostei have for some time been a source of great difficulty to morphologists; and any contributions with reference to the ontogeny of these structures are of interest.
The essential point which we have made out is that the anterior part of the oviduct of Lepidosteus arises by a fold of the peritoneum attaching itself to the free edge of the genital ridge. We have not, unfortunately, had specimens old enough to decide how the posterior part of the oviduct is formed; and although in the absence of such stages it would be rash in the extreme to speak with confidence as to the nature of this part of the duct, it may be well to consider the possibilities of the case in relation to other Ganoids and Teleostei.
The simplest supposition would be that the posterior part of the genital duct had the same origin as the anterior, i.e., that it was formed for its whole length by the concrescence of a peritoneal fold with the genital ridge, and that the duct so formed opened into the segmental duct.
The other possible supposition is that a true Müllerian duct—i.e., a product of the splitting of the segmental duct—is subsequently developed, and that the open end of this duct coalesces with the duct which has already begun to be formed in our oldest larva.
In attempting to estimate the relative probability of these two views, one important element is the relation of the oviducts of Lepidosteus to those of other Ganoids.
In all other Ganoids (vide Hyrtl, No. II) there are stated to be genital ducts in both sexes which are provided at their anterior extremities with a funnel-shaped mouth open to the abdominal cavity. At first sight, therefore, it might be supposed that they had no morphological relationship with the oviducts of Lepidosteus, but, apart from the presence of a funnel-shaped mouth, the oviducts of Lepidosteus are very similar to those of Chondrostean Ganoids, being thin-walled tubes opening on a projecting papilla into the dilated kidney ducts (horns of the urinary bladder, Hyrtl). These relations seem to prove beyond a doubt that the oviduct of Lepidosteus is for its major part homologous with the genital ducts of other Ganoids.