PORIFERA SPONGES. 



cells— the blastula. A few cells at the lower pole remain large, and are 

 filled with nutritive granules ; the other cells divide rapidly and become 

 small, clear, columnar, and flagellate. The large granular cells become 

 temporarily invaginated, forming what is called a " pseudo-gastrula." 

 This leaves the parent and the granular cells right themselves, forming 

 the posterior hemisphere of the embryo, now called an amphi-blastula. 

 It swims for a time actively, but the flagellate cells of the anterior 

 hemisphere are invaginated into or overgrown by the large granular 

 cells, and thus what is generally called the gastrula stage results. This 



soon settles down, on rock or seaweed, 

 with the blastopore or gastrula mouth 

 downwards, and is moored by 

 amoeboid processes from the granular 

 cells, which likewise obliterate the 

 blastopore. The granular cells lose 

 their granules, for the larva is not 

 yet feeding ; the now internal flagella 

 disappear in the absence of the 

 stimulating water ; a mesoglcea with 

 spicules begins to be formed between 

 the inner and outer layer, probably 

 by migrants from the latter. But 

 this disadvantageous state of affairs 

 cannot last. Pores open through the 

 walls, the entiance of water enables 

 the inner cells to recover their 

 flagella, and an exhalant aperture is 

 ruptured at the upper pole. The 

 young sponge is now in an Ascon 

 stage, from which, by the outgrowth 

 (?) of the inner layer into radial 

 chambers, it passes into the permanent 

 Sycon form, grows into a cylinder, 

 and becomes differentiated in detail 

 (Fig. SO- 



(b) In Oscarella (Halisarca) lobul- 

 aris (Fig. 52), a sponge without any 



-gr.c. 



am.p 



Fig. 5T. — Development of Sycandra 

 raphanus. — After F. E. Schulze. 



1. Ovum. 



2. Section of 16 cell stage. 



3. Blastula with 8 granular cells (gr.c.) 

 at lower pole. 



4. Free-swimming anvphiblastula, with 

 upper hemisphere of flagellate cells 

 (f.c.\ and lower hemisphere of gran- 

 ular cells. 



5. Gastrula stage settled down. Ec, 

 outer layer (ectoderm ?) ; En. , inner 

 layer (endoderm ?) ; 61., closing 

 blastopore; am.p., mooring amoe- 

 boid processes. 



