252 LILIAN SHELDON. 



yolk-spheres should suddenly change their properties towards 

 staining reagents. The only other possible mode of origin for 

 them is from the nuclei, which are present throughout the egg 

 at the stage before these bodies are present. If this is the 

 correct solution the nuclei must have been broken down and 

 considerably altered before they were converted into their 

 present form, since in the latter they are much smaller, instead 

 of being granular they are homogeneous and highly refractive, 

 and they stain much more deeply. Whatever their origin may 

 be, they are undoubtedly an important factor in the nutrition 

 of the egg, as they are found very plentifully scattered in the 

 ectoderm in the comparatively early stages, and are afterwards 

 absorbed without leaving a trace. 



Mode of Formation of the Posterior End of the 

 Embryo. — As was said in the descriptive part of this paper, 

 I am unable to state with certainty the exact method by which 

 the posterior end of the embyro is formed out of the egg; but 

 however this may be eflPected, the condition attained in which 

 the yolk contained in the anterior and posterior portions are 

 separated only by a single thin layer of protoplasm is very 

 remarkable, since at that stage the embryo possesses no definite 

 ventral ectoderm, the ventral surfaces of the anterior or poste- 

 rior halves being so closely applied to one another that the 

 single protoplasmic septum belongs equally to each, and cannot 

 be referred to one more than to the other. 



In no other known type of development, so far as I know, 

 does any process similar to this occur. It seems to have been 

 acquired as a part of the peculiar crystallising-out process 

 mentioned before as constituting one of the characteristic fea- 

 tures in the development of this creature. It is no doubt a 

 simple method for the formation in situ of the embryo, since 

 it involves no doubling or growth in length within the egg ; 

 also, owing to the large space occupied by the peripheral 

 nutritive layer, the amount of room within the egg is limited 

 at this stage, and it would not be possible for the embryo to 

 grow to any extent in length, so that the production of a 

 doubled-up embryo in situ from a single embryonic mass 



