14 EMBRYOLOGY 



The polar globules of the sponges have been overlooked up to the pre- 

 sent time. According to recent observations by Magdeburg, not yet 

 published, they present in Placina the appearance typical for most of the 

 other Metazoa (Fig. 1 i'). Also the processes of their formation may 

 well find place in the general plan, while, according to Fiedler's com- 

 munication {Zeitschr. f. loiss. Zool., Bd. 47) on Spongilla, it would almost 

 seem as if there existed here a peculiar type of formation. 



(3) The eggs undergo total cleavage, and develop in the 

 parent body into spheroidal or ovate embryos covered on the 

 surface witli flagella. 



(4) When the embryos have readied the stage of the 

 oval, flagellate, so-called planula larva, they emerge and pass 

 through a swarming stage, during which development makes 

 but little progress. 



(5) After attachment to a fixed support is effected there 

 follows a rapid transformation into a young sponge, resem- 

 bling substantially the parent. 



We may best arrange the types of sponge development 

 hitherto known in accordance with the characteristic con- 

 dition of the swarming stage. 



I. — Type of development through a so-called 

 Amphiblastu la-stage. 



The development of Sycandra raphanus, which has been 

 described by Metschnikoff (Nos. 12 and 13) and F. E. 

 ScHULZE (Nos. 19 and 22), serves as an example of this type. 



The egg of this calcareous sponge undergoes a total and 

 nearly equal cleavage, but the course of cleavage is somewhat 

 modified by the relation which the embryo acquires to the 

 wall of one of the radial tubes of the parent (Fig. 2). 



The egg is a naked cell, and lies in the parenchyma close 

 to the wall of a radial tube. It is first divided into two 

 blastomeres of equal size (Fig. 2 A) by means of a furrow 

 which is perpendicular to the radial tube, and in relation to 

 the orientation of the developing embryo must be con- 

 sidered as meridional. By means of another meridional 

 furrow perpendicular to the first one, the two cleavage 

 spheres separate into four blastomeres, now arranged in the 

 form of a cross (Fig. 2 B), which are applied to the radial 



