No. 3-] DEVELOPMENT OF MARINE SPONGES. 295 



which it is composed are flat and of considerable size. Since 

 this layer forms the ectoderm of the adult, and is a perfectly 

 well marked embryonic layer, it seems justifiable to speak of it 

 as the ectoderm. In the gemmule shown in this figure (Fig. 

 22) the separation of the gemmule masses into their constitu- 

 ent cells has nearly reached its end. There are still left some 

 small multinucleate masses {m. n. tn.), in which the cell bound- 

 aries are difficult, in many cases impossible, to make out. But 

 the bulk of the gemmule is now composed of distinct cells. 

 The mass of cells inside the ectoderm will be spoken of as the 

 parenchyma or mes-entoderm, for from it are formed all the 

 tissues of the adult except the epidermis. The cells of the par- 

 enchyma are abundantly provided with processes which connect 

 with one another, and so establish an intercellular network. 

 The multinucleate masses also have delicate processes, like 

 those of the separate cells, and form part of this network. 



The gemmule shown in Fig. 23 represents a slightly older 

 stage than Fig. 22. The ectoderm cells have increased in 

 number and have become smaller. The multinucleate masses 

 of the earlier stage have broken up into separate cells. There 

 are found here and there karyokinetic figures in the cells of 

 the parenchyma. Hitherto the gemmule has been spheroidal, 

 but now it begins to assume an oval shape. 



With the assumption of a definitely oval shape there goes 

 hand in hand a differentiation of the poles, PL XVI, Fig. 24. 

 The ectoderm cells steadily increase in number, becoming all 

 the time narrower and gradually elongating. In this way the 

 cells become very long and slender, and the nuclei come to be 

 arranged in several layers. In Fig. 24 the nuclei are just 

 beginning to abandon their original arrangement in a single 

 layer, and in Fig. 25 the transformation of the ectoderm cells 

 is nearly complete. When the ectoderm cells have taken on 

 the columnar shape, an orange pigment is deposited in their 

 peripheral ends, and each cell develops a flagellum. Over one 

 pole, however, the pole {p.p. in Fig. 25) which is to be the 

 hinder one in swimming, the ectoderm cells do not become 

 columnar. At this pole the parenchyma cells accumulate in 

 such numbers as to form a dense mass, p.p., Fig. 24. This 



