216 SCIENCE PROGRESS. 



interior of the embryo is composed, at an early stage, of 

 a uniform mass of granular macromeres, each with a vesi- 

 cular nucleus. The first differentiation is that some of 

 these become clearer and less granular, and commence 

 to secrete spicules. These " scleroblasts " still retain, how- 

 ever, the vesicular nucleus. A part of the remaining 

 blastomeres then becomes transformed into cells with 

 uniform, finely granulated protoplasm and a nucleus con- 

 taining a framework and evenly distributed chromatin ; 

 these cells are destined to give rise to epithelial, con- 

 tractile and connective tissue cells. In addition there 

 remain over a number of undifferentiated blastomeres, 

 with coarsely granular protoplasm and vesicular nucleus. 

 These are set apart to become the future amoeboid and 

 genital cells of the adult sponge, so that we have here 

 an interesting and striking case of the germ cells being 

 separated at an early period from the somatic cells and 

 retaining the primitive characters of the blastomeres of 

 the ovum. Hence the inner mass of the larva consists of 

 (i) scleroblasts with spicules, the latter always having a 

 definite arrangement, which in some species {e.g., Esperia 

 Lorenzi) may attain to a high degree of complication ; (2) 

 undifferentiated cells, corresponding to Delage's amceboid 

 cells ; and (3) differentiated cells, corresponding to Delage's 

 "cellules epidermiques" and " intermediates " — a distinc- 

 tion which Maas does not recognise. Some of these 

 differentiated cells, when bounding a free surface, as at the 

 hinder end of the larva, take on an epithelial arrangement, 

 but the cells so modified are not to be regarded as 

 essentially different from those lying beneath them ; their 

 difference in appearance is due to the accident of their 

 position, so to speak. Other differentiated cells again are 

 often arranged radially or tangentially in the larva, and 

 probably serve for the contraction of the body. In spite 

 of the different elements it contains, Maas considers the 

 inner mass as forming but a single germinal layer, and as 

 a proof of this he points, on the one hand, to the destination 

 of its cells in the adult sponge, and, on the other, to its 

 origin from a uniform mass of macromeres in the embryo. 



