DEVELOPMENT IN Pr,T^AIl'T. \Rr\. ()l 



The embryo continues to s:i'ow, and the cells of which it is 

 composed increase ^s^reatl}' in number, but their size remains the 

 same as before. The endoderm of the embryo is continued 

 rii;ht round antl liecomes more than one cell thick. The ecto- 

 derm is also mostly two or three cells in thickness. The cells of 

 the ectoderm and endoderm differ but little from each other in 

 appearance. Init the endoderm cells are perhaps a little flatter. 

 There is, however, a fairly sharp line of division (an !nci|)ient 

 mesoglea) between the two layers. Whether the endoderm 

 arises as a definite splitting off of the ectoderm (delamination) 

 or by a .g^radual differentiation of the innermost cells cannot be 

 clearly seen, but the difference between the two method^ of 

 orio^in is probably so small that it may be regarded as an artificial 

 distinction which cannot be fulb' distinguished in anv individual 

 case of development. 



The outermost cells of the ectoderm of the embrvo are not 

 sharjily marked off from the placental tissue. The cells of this 

 tissue come into the most intimate connection with the embryo. 



The embryo still further increases in size and pushes itself 

 into a more central position in the gonotheca. The ccelenteron is 

 very extensive, and the wall n{ the embrvo is comparativelv 

 very thin. This wall is thinnest along the flattened sides of the 

 gonotheca, and here it consists of a single layer of ectoderm and 

 endoderm cells, and is closely .squeezed against the perisarc. 



Above, below and laterally, the wall is considerablv thicker, 

 arid both the ectoderm and the endoderm are several cells in 

 thickness. Thus in section parallel to the plane of flattening of 

 the gonotheca the planula is quite thick walUyl. 



The material has not permitted an examination of a later 

 stage, but it is probable that the planula in the above condition 

 i^ nearlv readv to break out of the gonotheca. for the gonotheca 

 that was immediately below, and tlierefore slightly c Ider, was 

 empty and the operculum gone, doubtless through th--' i)iu-sting 

 out of the planula. 



Here, then, wc liave an interesting case of the nourishing of 

 the developing embrvo. Tn the majoritv of Invertebrates the 

 egg is charged with enough food or volk-material to enable 

 development to proceed sufficientlv far until the voung creature 

 c?n feed and provide itself with food. Tn this hydroid the e^;^ 

 rcma^ins quite small and is never i^rovided with volk. but it 

 segments, and development takes place in a kind of placental 

 maternal tissue, which supplies the embrvo with food during the 

 whole develoi)ment. This placental tissue arises as a modifica- 

 tion of an outgrowth of the ectoderm of the blastostvle. This 

 outgrowth, as we have seen, forms a kind of cap over the young 

 ovum, and doubtless may be regarded as representing a rudimen- 

 tary gonophore. 



