194 J. S. NICHOLAS 



cells themselves or which cause them to do certain things. It gives a fate 

 map which, while telling the generalized movement of parts, tells noth- 

 ing about their specific mechanism of differentiation. For this phase of 

 study we are dependent upon other methods, all of which bring about 

 indirectly other effects which sometimes obscure the action of the ma- 

 terial itself, or the action of the system immediately surrounding it. 

 This is true of a great many of the transplantation or isolation studies 

 which, while they give interesting pictures of what a cell can do, tell 

 little concerning its reaction in its own specific pattern with regard to 

 its own specific organization. Furthermore, some materials can be 

 mapped less exactly than others. For instance, the teleost blastoderm 

 lacks pigment which is responsible for the maintenance of a persistent 

 color value. Stains likewise fade more rapidly in the teleost than in the 

 amphibian. The same is true in avian studies where the reduction of 

 dye sometimes leads to exceedingly erroneous interpretations. Extirpa- 

 tion or isolation of parts which are to be studied has given quite differ- 

 ent results from those secured when the same parts are integral com- 

 ponents included within a migrating tissue under normal circumstances 

 and surroundings. It is evident that some change occurs in the early 

 development of the fish egg which is responsible for later developments, 

 since an isolated blastodisc can continue development when isolated 

 after the 8- or i6-cell stage, but before that time it is incapable of 

 adjusting itself to the new environment in such a way as to give a com- 

 plete embryo formation. Parts may be formed but the total with its 

 organization is not consistently present. Tung, Chang, and Tung ('45) 

 have noted exactly similar conditions in the goldfish egg. 



Another factor which seems to have received too little attention is the 

 egg-contained yolk. It constitutes a fundamental problem for each 

 embryo, for the developing structures are molded around the generally 

 spherical yolk content of the egg. The importance of this mass which 

 presents in many forms a surface around which the embryo must grow 

 was pointed out in a paper by Daniel and Yarwood ('39). They studied 

 the distribution of the yolk fragments in various stages of development 

 of the egg using coelomic, ovaducal, and fertilized eggs and compared 

 the stratification of the yolk platelets in each of these stages. 



In the sectioned material there is a practical uniformity of distribu- 

 tion at the stages that were surveyed. Studies on later stages show that 

 there is practically no diminution in the size of the yolk platelets until 

 after it is formed in the particular cell in which it is destined to act as 

 a metabolite. Yolk platelets in different systems of the embryo are all 



