EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 65 
of the blood-producing mesenchyme, still de facto he is 
quite outspoken about it. His conclusion (I. ¢., p. L095) that 
it is not of any importance, whether the blood anlages of the 
Selachians are meso- or entoblastic is not ours, because we 
have in the first part of this chapter demonstrated that a 
precise analysis of the mesoblast formation also in mammals 
may considerably contribute to render comparison of the 
various classes more easy. That in Elasmobranchs, as in 
mammals, a very early fusion comes about between Riickert’s 
peripheral and axial mesoblast, and that, after this has come 
about, an end is made to further unravelling is certainly 
true, but it may not withhold us from laying all the more 
stress on the necessity of comparing the very earliest stages 
with the utmost closeness. 
The focus of proliferation corresponding to what we have 
called the ventral mesoblast in the Mammalia and _ the 
Amphibia must be. sought for in the Hlasmobranchs in the 
tail swellings. I do not intend to enter into further compari- 
son here, but will postpone this to a later paper where the 
stages after the development of the mesoblastic somites will 
be discussed. 
Among the Teleostomes we find developmental stages 
which have again a different character from what we noticed 
in Hlasmobranch fishes. In many cases the eggs are not mero- 
blastic, and then a more or less close comparison is_ possible 
between their development, and that of the Amphibia above 
discussed. So it is with the Sturgeon’s eggs and with those 
of the Dipnoi: Ceratodus, Lepidosiren, and Protopterus (Fig. 
125). Other comparisons with the development of the 
lamprey suggest themselves, and have already been pointed 
out by different authors. As to the external features of this 
development, the Riickenrinne, which Semon has figured for 
Ceratodus (Fig. 126), and which has also been noticed in 
Urodeles by Braus seems to me to be simply explained if we 
look upon it, not as any remnant of a blastopore (Urmund), 
as Semon (93, pp. 37—839) proposes, but as the last reminisc- 
VOL. 53, PART 1,.—NEW SERIES. 5) 
