FORMATION OF BLASTODERM 177 



first and the ovum subsequently formed round it. But the 

 ovum was not, in Schwann's view, a simple cell, for within it 

 were contained yolk-granules, one set apparently containing 

 a nucleus, the others not. Even the second set, those 

 composing the yellow yolk, were considered by Schwann to 

 deserve the name of cells, because, although a nucleus could 

 not be observed in them, they had a definite membrane, 

 distinct from their contents a conception of the cell obviously 

 dating from the earliest botanical notions of cells as little 

 sacs. The yolk cells were not mere dead food material but 

 living units which took part in the subsequent development 

 of the egg. The relation between the unfertilised egg and 

 the blastoderm which arises from it is not made altogether 

 clear by Schwann. According to his account the cells of 

 the blastoderm are formed actually in the ovum. Round the 

 nucleus of the egg appears a Niederschlag or precipitate 

 which is the rudiment of the blastoderm (p. 68). When the 

 egg leaves the ovary the nucleus disappears, leaving behind 

 it this rudiment of the blastoderm, which rapidly grows and 

 increases in size. The blastoderm of the chick before 

 incubation is found to be composed of spherical anucleate 

 bodies which Schwann considers to be cells, because they 

 almost certainly develop into the cells of the incubated 

 blastoderm, which are clearly recognisable as such after eight 

 hours' incubation. The serous and mucous layers can be 

 distinguished after sixteen hours' incubation, and it is found 

 that the cells of the serous layer contain definite nuclei, 

 though such seem to be absent in the cells of the mucous 



o 



layer. Between the two layers other cells are formed 

 belonging to the vessel layer, which is, however, in Schwann's 

 opinion not a very definitely individualised layer. 



Schwann's next step is a detailed demonstration of the 

 origin of each tissue from simple cells such as those composing 

 the incubated blastoderm. 



" The foregoing investigation has taught us that the 

 whole ovum shows nothing but a continual formation and 

 differentiation of cells, from the moment of its appearance 

 up to the time when, through the development of the serous 

 and mucous layers of the blastoderm, the foundation is 

 given for all the tissues subsequently appearing: we have 



