The early development of Eudendrium. 269 



Formation of the Ectoderm. 



Attention has already been directed to the migration of nuclei 

 from the midst of the yolk spheres toward the periphery of the egg. 

 Here an indefinitely reticulated ectosarcal layer of cytoplasm early 

 appears and within this layer the nuclei find their places, increasing 

 in numbers, both by the process of migration as well as by free 

 multiplication, till a fairly well distributed series is formed over the 

 entire surface, as shown in Figs. 18 — 20, tho there are as yet no signs 

 of distinctive cell boundaries or cell individuality. As will be noted 

 from an inspection of the figures just cited, as well as from several 

 others of slightly later stages, this peripheral layer does not at this 

 time constitute a definite ectoderm, lacking the histological characters 

 of a distinctive tissue, as already intimated, or at any rate the 

 distinctive features of true ectoderm. That it does constitute an 

 ectodermal border, or probably, an ectodermal anläge, does not seem 

 in the least doubtful, and may therefore justify the designation of 

 primitive ectoderm. In view of the more or less marked changes 

 which occur in its development the latter might seem upon the whole 

 the more appropriate designation. 



It should be noted in this connection that the protoplasm of this 

 layer is in some cases rather granular, in others more reticular, and 

 often in both conspicuously vacuolated. This last feature is more or 

 less evident throughout the entire course of development, becoming 

 more so' as the ectoderm assumes final form. 



As the nuclei increase in number they come to form a more or 

 less symmetrical series, giving to the layer the aspects of a distinct 

 tissue though as yet without evident cell-boundaries. 



In the earlier phases of development the nuclei, as well as 

 the cytoplasm , of the primitive ectoderm differ considerably in 

 size as well as in number and the thickness of the cytoplasmic 

 border. This will be evident from an inspection of the figures just 

 cited. With the progress of development these differences diminish 

 but do not wholly disappear, even in the mature embryo or free- 

 swimming planula, as will be further shown later on, nor is the 

 development of the ectoderm uniform over the entire embryo. It 

 may often be seen to be fairly well differentiated at one pole, usually 

 that destined to become aboral, while it remains for some time in- 

 definite at the opposite pole. 



