BLASTOPORE, MESODERM AND METAMERIO SEGMENTATION. 15 
Blastopore, Mesoderm and Metameric 
Segmentation. 
By 
W. II. Caldwell, M.A., 
fellow of Caius College, and Balfour Student in the University of Cambridge. 
With Plate II. 
A year ago my observations on the development of certain 
forms of invertebrate animals suggested an explanation of the 
behaviour of the blastopore, and led me to consider the various 
speculations concerning the middle germinal layer put forward 
in recent years. The theory which I thus deduced embraces the 
question of metameric segmentation. On my voyage to Australia 
I wrote the present paper. Meanwhile my friend Mr. Sedgwick, 
to whom I had written some of my conclusions, was preparing 
a contribution to the same subject. Since then Mr. Sedgwick's 
paper 1 has appeared, and I find not only that my conclusions as 
to the meaning of segmentation are fundamentally different, 
but also that he leaves the larger question of mesoderm un- 
touched. The origin of mesoderm in Phoronis was the starting- 
point of my inquiry. I shall first describe the facts in this 
animal, and I would point out that my discoveries are due 
entirely to the facilities for investigating minute embryos 
afforded by my method of obtaining automatic series of sections. 
I failed to observe the details which are contained in the present 
paper in my original sections from which a preliminary account 
(‘ Proc. Roy. Soc. 5 1882) was composed. 
1 ‘ Quart. Journ. Mic. Sci.,’ 1884, “ Origin of Metameric Segmentation.” 
