32 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Progressive Differentiation of Cells in the Course of Development.*' 
— Prof. Nussbaum begins his discussion by noting that no amount of 
external influence will rear a duck out of a hen’s egg. Ova are 
specialised. Yet as recent work shows, they can be modified not a 
little ; experiment must decide the limits of this modifiability. Simi- 
larly, at different times, and to different degrees, somatic cells become 
specialised. According to Pfliiger, the frog’s ovum is isotropic, either 
the dark or the white half may form the central nervous system. 
According to Roux, the head and tail ends are defined by the point 
at which the spermatozoon enters. The results reached by Driesch, 
Wilson, and others point to relative isotropy. But at a certain stage, 
sooner or later, specialisation occurs ; certain layers of cells can only 
form certain organs. This is illustrated in reference to the development 
of the eye and other organs. But here again there is great plasticity, 
the limits of which experiment must define ; and Nussbaum refers to 
various regeneration-experiments which we have from time to time 
recorded. He believes in gradual loss of many-sidedness on the part of 
the somatic cells, in differentiating cell-division, in the course of which 
functions and potentialities are restricted. And this restriction is in 
proportion to the grade of phyletic and ontogenetic development. 
Metamerism of Vertebrate Head.f — Mr. A. Sewertzoff gives an 
account of the development of the occipital region of the Lower 
Vertebrates, in connection with the question of the metamerism of the 
head. He comes to the conclusion that there was a time when the head 
of the ancestors of existing Vertebrates was segmented. The metameres 
of the head were in their essential characters similar to the metameres 
of the trunk. They consisted of myotomes, and were innervated by the 
ventral roots of spinal-cord-like nerves. There exists a complete series 
of intermediate stages between typical head-segments and typical trunk- 
segments ( Acijpenser ). The relation of the head-myotomes of the head- 
nerves, for example, to the vagus nerve, is the same as that between 
trunk-myotomes and the spinal nerves. So like are the head-segments 
to the trunk-segments that it is difficult to recognise any fundamental 
distinction between them. The resemblance between the several somites 
of the head in different groups of Vertebrates is due to this. The 
somites which lie in front of the second somite of the metaotic region 
of the Urodeles are segments common to all the Craniata, and are 
consequently primitive head-myotomes. Succeeding segments which 
lay immediately behind them, and primitively belonged to the trunk, 
gradually became parts of the head ; first the occipital arch and the 
myotome which lies in front of it. The Urodeles have remained at 
this stage. In the Anura another myotome is annexed. Finally, w r e 
have in Fishes, Sauropsida, and Mammalia, the most complete expression 
of this process, so that in them a whole series of trunk- segments has 
entered into the constitution of the head. The evolution of the hinder 
part of the head has therefore followed the disappearance of the meta- 
merism which extends from before backwards, and to the gradual 
conversion of the anterior segments of the trunk into the hinder part of 
the head. 
* SB. Niederrhein. Gesellsch. Bonn, 1894, pp. 81-94. 
t Bull. Soc. Imp. Nat. Moscou, 1895, pp. 186-284 (2 pis.). 
