18 T. H. MORGAN. 



From this stage we pass to embryos of the age represented by- 

 Figs. I and II, Plate IV". The first figure, 18, is a cross-section 

 through the stomodseum of Fig. I (see line 18 of the figure). The 

 invagination is deeper than in the last section, and its lumen is 

 closed. The ectoblast nuclei of its walls are two layers thick. On 

 each side of this central invagination is what appears to be a lateral 

 invagination. These supposed invaginations have given me end- 

 less trouble, and even now I feel some uncertainty as to my 

 interpretation of them. On each side of these is seen the thick- 

 ened ectoblast of the first pair of appendages. The lateral in- 

 vaginations are, I believe, caused by the growth of these append- 

 ages, which tend to grow outwards as they increase in size ; but 

 are prevented from doing so by the egg coverings, and the result 

 is that the ectoblast becomes folded on itself to make room for 

 the growing appendages. This view is strengthened by the pres- 

 ence of somewhat similar invaginations at the side of the other 

 appendages. This part which is pushed in would correspond to 

 the dark furrow between each appendage of Fig. I and the cells 

 forming the stomodseum. Themesoblast around thestomodseum 

 and under the appendages has increased, and is now clearly dis- 

 tinguishable from the inner covering of the entoblast, which lies 

 only at the periphery of the yolk, and between it and the meso- 

 blast. 



In Fig. 19 we have a section from the same series as the last, 

 but more anterior and through the region of the brain correspond- 

 ing to line 19 of Fig. I. The section is entirely in front of the 

 stomodseum, and cuts the two brain thickenings of the surface 

 view. Here again the ectoblast is seen to be distinctly folded, 

 and this folding (invagination !) is directly continuous with the 

 last. At first sight this seems nothing more than a forward con- 

 tinuation of the last groove, but it is not clear why the folding of the 

 appendage should exert any influence over that part of the brain. 

 Again, it might be interpreted as folding due to the brain, but I 

 can see no good reason why such thickenings (not outgrowths) 

 should produce the groove. There is one other possibility, viz. 

 that these may represent invaginations into the brain itself. 

 Against such a view is the absence in surface views of any such 

 invaginations, and also that the folds are directly continuous with 

 and quite similar to the groove between the stomodseum and the 



