Mosaic Dcvelopmcjit in Ascidian Eggs. 209 



VI. GENERAL CONCLUSIONS. 



The conclusions which follow from these experiments are so 

 obvious that they need but little emphasis here. Not only is the 

 fact established that individual blastomeres give rise only to those 

 parts of an embryo which they would produce under normal 

 conditions, but the cause of this is clearly indicated. The devel- 

 opment of the ascidian egg is a mosaic work because individual 

 blastomeres are composed of different kinds of ooplasmic material; 

 this mosaic work is not merely a cleavage mosaic but also a mosaic 

 of germinal substances, several of which are recognizable before 

 cleavage begins. 



I. Organ-Forjning Substances. 



I have elsewhere shown that at least five distinct kinds of 

 ooplasm are recognizable in the egg of Cynthia before the first 

 cleavage and that all of these substances are localized in their 

 final positions as early as the close of that cleavage. In these 

 experiments I have not been able to isolate the different ooplasmic 

 substances in the unsegmented egg, but after the second or third 

 cleavages several of these substances may be isolated and in such 

 cases each substance gives rise only to a definite kind of tissue or 

 organ, and apparently it has no power to produce any other kind. 

 The myoplasm produces muscle cells only; the chorda-neuro- 

 plasm, only chorda and neural plate cells; the chymoplasm, only 

 mesenchyme; the endoplasm and ectoplasm only endoderm and 

 ectoderm, respectively. Whenever an isolated blastomere lacks 

 any of these substances, the embryo which develops from that 

 blastomere lacks the corresponding organs. Accordingly the 

 potencies of individual blastomeres are dependent upon the ooplas- 

 mic substances which they contain; the prospective value of any 

 blastomere is not primarily a function of its position, but rather 

 of its material substance. 



The reason that the anterior quadrants of the egg never produce 

 muscle cells is evidently due to the fact that they totally lack the 

 yellow myoplasm; the fact that the posterior quadrants never 

 produce a neural plate or chorda, is evidently due to the complete 

 absence of the chorda-neuroplasm in these quadrants; the cells 

 of the ventral (animal) pole produce only ectoderm, without a 

 trace of endoderm or mesoderm, — evidently because these cells 

 are composed almost entirely of clear ectoplasm. 



