SOME TEACHINGS OF DEVELOPMENT. 
209 
as to overlap the second one representing the gastrula, and in 
the case of Ciepsine so as almost to reach the base upon which 
the telescope rests. 
II. 
At the end of the previous lecture we had arrived at the con- 
sideration of what we, for convenience, regarded as the third 
stage in the developmental staircase, the third tube in the de- 
velopmental telescope — the formation, namely, of the mesoderm ; 
and I insisted that the term is a misnomer, because it obviously 
places itself side by side in the mind with the names which have 
long been applied to the two primary layers, ectoderm’^ and 
entoderm,” whereas it is secondary to these, being derived 
from them. The mesoderm consists, in fact, of cells which 
have been separated or segregated from amongst the cells of the 
primary layers for the performance of special functions. In the 
lowest animals in which such a separation occurs it may take 
place at any part of the primary layers, or even over the whole 
of their extent. We see this in the sponges and jelly-fish. In 
these, when intermediate cells are separated, they lie in a jelly- 
like substance, and the main purpose which the segregation sub- 
serves is that of support and connection. Now, it is obvious 
that this sustentacular function might be almost as well per- 
formed by the inert jelly alone — in fact, the soft protoplasm of 
the cells can be of but little assistance, so that we should natu- 
rally look upon the cells in this primitive mesoderm rather as 
ministering to the nutrition of the jelly than as agents in the 
performance of its function. This view is strengthened by ob- 
serving that it is the intermediate jelly-like matrix of this primi- 
tive connective tissue which is the first to appear, the cells (when 
they do occur, for in many cases they are absent throughout life) 
wanderiog into it subsequently. In the larger Medusae (Aurelia) 
they come, according to the testimony of most observers, from 
the entoderm ; in the Sponge they are derived from the outer of 
the two layers which are found in the Olynthus stage — the one 
which is generally regarded as ectoderm.^ 
^ There seems to be some uncertainty about the interpretation of the 
two layers of cells which are found in the Olynthus stage of development 
of the Sponge. According to the descriptions of various observers, and 
especially that given by F. E. Schulze with regard to Sycandra raphanns, 
the segmentation of the ovum takes place, and a blastosphere becomes 
formed much in the usual manner. Of the cells of this blastosphere 
those of one hemisphere are smaller clearer, and provided with long cilia, 
those of the other and lesser hemisphere being larger, more granular, 
and without cilia. Presently the latter become invaginated, but the 
cupping thus produced proves a temporary condition merely ; fluid again 
accumulates in the cavity of the blastosphere and obliterates the cup- 
