DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 531 



ning to flatten or thin out in order to begin its expansion to 

 cover the yolk sphere. After the fourth or fifth cleavage some 

 of the peripheral cells of the germ disc are somewhat fused with 

 the yolk mass and do not present a clearly formed distal cell wall. 

 The nuclei of such cells continue to divide and begin to wander 

 or are pushed out into the superficial yolk material. In this 

 way are formed the so-called periblast nuclei, or more correctly 

 periblast syncytium, of the teleost. This periblast syncytium 

 precedes the germ disc in its descent over the yolk, so that one 

 observes loosely scattered nuclei of unusually large size forming 

 an advance border around the periphery bf the germinal disc. 

 The nuclei multiply and finally lie scattered over the entire yolk 

 surface by the time the genu ring or blastodisc has completely 

 covered the yolk (figs. 5, 7 and 8). 



These periblast nuclei are of interest to us in the present con- 

 sideration only on account of the fact that they are located 

 in a superficial syncytium covering the yolk. It is over this 

 syncytium that the mesenchymal cells wander. The periblast 

 of the hypoblast-free yolk-sac of the teleost, in so far as position 

 is concerned, may be compared to the endodermal covering of 

 the yolk-sac in other meroblastic eggs. 



The outer cover of the yolk-sac in Fundulus is formed by the 

 germinal disc as it grows over the yolk. This constitutes the 

 ectoderm of the sac which is its only true or typical layer. Thus 

 the yolk-sac consists of an outer-continuous ectodermal layer 

 beneath which are freely wandering mesenchymal cells and below 

 these the periblastic syncytium fuses into the yolk material 

 itself. 



The periblast nuclei were interpreted by Agassiz to represent 

 the survival of the nuclei which had at one time in phylogeny 

 controlled the segmentation of the yolk. These were the nuclei 

 of the former yolk laden cells in the holoblastic cleavage of the 

 ancestral teleost. Others have thought that they played some 

 part in the formation of the ventral wall of the gut, etc. In 

 Fundulus, however, they take no part in the formation of the 

 body tissues or organs, but may be observed to degenerate in the 

 late embryo . The periblast nu clei become very mu ch vacu olated. 



