13 



enlargement, lying in Iront ol' the foremost in tig. (J ; the contents and walls, however, of all these brain vesicles are so 

 misshapen that no distinct cellular elements can he distinguished. While it is scarcely possible, from the specimen under 

 consideration, to declare with certainty whether this laterally-situated brain vesicle enlai'gement in the fore part of the medullary 

 tube is the primary anterior or the secondary anterior brain vesicle, the cerebral hemispheres vesicle, or possibly the not yet 

 invaginated optic vesicle (which, however, seems to me to be very improbable), I believe that the foremost enlargement of the 

 medullary tube in iig. (5. cb. should be considered as the mesencephalon. The elongated part lying in the rear would then 

 h.nve to be taken for the myelencephalon and its continuation in the spinal cord. 



If that part of the mesoderm tissue in the brain region, which lies on the ventral side of the medullary tube be 

 carefully examined, a dorsally-i)roj?cting, conical process is at once apparent, forming the posterior boundary of the mesencephalon, 

 and separating it from the parts lying behind. This ]n-ojection I consider ought to be interpreted as Kolliker's hindmost 

 "Schadelbalken", while one in front of the foremost brain vesicle enlargement in fig. 6 ought to be taken for Kolliker's 

 "vordere Schadelbalken"' (Rathke's "mitlere Schildelbalken"). This explanation is supported by the numerous proto-vertebræ 

 lying behind, which attach themselves entirely to the rest, only that they lie somewhat close together, and that directly below 

 (ventrally) lie the aorta and heart, so that we are only justified, from these relations, in considering that part of the medullary 

 tube lying above as corresponding to the neck region. 



The fact that the proto-vertebræ, the chorda dorsalis and the medullary tube appear in the various sections with 

 a maximum and minimum of distinctness in ditierent places, is owing to the subjection of the embryo's axial skeleton with 

 the medullary tube to side compression. The median line of the foremost brain vesicle is thus met with long before the 

 hindmost, and about simultaneously with the median line of a part of the back. There is thus without doubt a double sigmoid 

 curve of the long axis. Moreover, the sections do not fall exactly in the sagittal plane, but intersect it at an acute angle. 



The integuments (PL I. tig. 8) show a purely embryonal structure, namely, the epidermis like a single-layered, 

 low, cubical epithelium, and the corium, as a very undifferentiated mesodermic tissue. 



The large, long fissure in the fore part of the embryo (fig. 6, F. t.) I interpret as the fore-gut. In one or 

 two laterally-situated sections, a few enlargements are observable, perhaps due to artificial compression. No mouth is to 

 be discovered. 



Below the fore-gut. a triangular region is seen (fig. 6, cor), which may be explained as the heart; in a few 

 sections the embryonal blood corpuscles can be seen in it: the walls are also visible. In some of the sections the vena 

 umbilicalis is visible, and can be traced from the umbilicus and seen to open, in front of the liver, into the heart. 



Behind the heart, separated by a bridge of mesoderm-tissue, is a larger oval object, to be recognised as the liver; 

 numerous blood-vessels are visible, and between these, large epithelial cells, which here and there form a lumen (Fig. 6 and 7 hep.). 



In a few lateral sections, beneath the proto-vertebræ, above a fissure (fig. 6, t) which I have interpreted as 

 being an intestinal lumen (?) can distinctly be seen the Wolffian bodies, with passages, numerous blood-vessels, and a 

 single glomerulus, although the whole example is very indistinct. 



Iii the description of the external form, there has already been mentioned an invagination situated ventrad of the 

 tail, and seemingly blind. This cavity can be traced in some sections to the side of the medial plane (fig. 7, a) where the 

 form of the anus is distinctly seen. The adjacent portion of the intestines is, on the other hand, not visible. The surrounding, 

 strongly-coloured, granulated parts here denote quicker cell-proliferation. 



The histological elements in this hinder, less injured part of the embryo possess, however, a certain peculiar 

 character which cannot be designated as quite normal. The boundary between the ditferent organs is slight, and the whole 

 example, as represented by the difierent sections, is somewhat obscure. 



If we sum up the discoveries now before us after the examination of this 8 millim. long embryo, they scarcely 

 admit of a doubt that it is an embryo at a very early stage of developement. Had it been entirely uninjured, the visceral 

 arches would very probably have been found, and the first slight indication of anterior extremities, while the perforation for 

 the mouth aperture could scarcely have been more than in its commencement. The external form of the trunk has certainly 

 been given with broad navel. The already long tail, although in its embryonal form, may yet be noted as characteristic. 

 In the embi'yo of a quadruped in a stage corresponding to this, bud-like indications of the extremities might well have been 

 expected; and similar conditions would also, in all probability, have been found here. As reliable points of comparison with 

 other animal embryos, there are the purely embryonal integument, the first indications of a cartilage formation, and the 

 scarcely complete segmentation of the extreme end of the tail. I must, however, characterize this embryo as abnormal, as it 

 has been undergoing a retrogressive or degenerative process. Certain portions of the fu'tal membranes also point in this 

 direction, as they show distinct advancement, while the embryo is in a state of retrogression. 



Having a short time ago obtained a very small embryo of Phoca'na, of which the form was remarkably well 

 preserved, and is to be described subsequently. I have been even more strengthened in my opinion that this 8 millim. long 



