192 Edivin G. Conklin. 



mesenchyme derived from the cell A^*^. In the embryo shown in Fig. 

 71 the neural plate cells are clearly shown around the periphery of 

 the figure and two of the cells contain sense spots. The chorda, 

 endoderm, and mesenchyme cells are shown internal to the neural 

 plate, but I am unable to distinguish in this embryo between these 

 three kinds of cells; they are all more or less yolk-laden as in the 

 normal egg. Owing probably to the fact that no ventral ecto- 

 derm cells are present the neural plate is not pushed up onto the 

 dorsal face and there are no evidences of gastrulation, although 

 normal embryos of a corresponding age have already reached the 

 full larval development. That this failure to gastrulate is not 

 due to the slower development of the egg fragments as compared 

 with the entire egg is shown by the degree of histological differen- 

 tiation of the neural plate and sense spots, the latter appearing 

 normally only in the fully formed larvae. 



Fig. 72 is a quarter embryo of the same age as the preceding, 

 derived from the cells A'^-^, a*-^ of the right anterior quadrant. 

 The ventral ectoderm cells have here pushed the neural plate cells 

 up onto the dorsal face of the embryo, w^hile the chorda cells (.?) 

 lie along the median and transverse furrows. Four sense spots 

 are present in the neural plate. 



Fig. 73 is also a quarter embryo of the same age as the pre- 

 ceding, derived from the two posterior ventral cells b*-, b^-. 

 This embryo consists entirely of ectoderm which is arranged in a 

 single layer of cells around a central cavity, the blastocoel. There 

 has been no gastrulation and the embryo contains neither endo- 

 derm nor mesoderm. A few of the ectoderm cells next to the 

 cell 'A*'^ contain yellow pigment, exactly as in the normal embryo. 



Figs. 74 to 76 are three views of one embryo, about 20 hours 

 after the egg was spurted in the 20-cell stage. By the spurting all 

 the cells were killed except seven from which this embryo has 

 developed, viz: a pair of mesenchyme cells B^-^, and five ectoderm 

 cells, b^% ^^-^ a-^ ^ a^^ and a^'^. (See Fig. 3.) This embryo consists 

 entirely of an outer layer of clear ectoderm cells, inclosing at its 

 posterior end a mass of small mesenchyme cells; it contains no 

 endoderm. It is an interesting fact that the mesenchyme cells are 

 here inclosed by the ectoderm, showing that some process in the 

 nature of gastrulation must have taken place. 



A great many other partial embryos, produced from one or 

 more blastomeres of the 8, 16 or 32-cell stages^ have been studied 



