xxvi GENERAL INTRODUCTION. 



of blastosphere 1 . The cells of the blastosphere are frequently (? always) 

 dissimilar at opposite poles, one set typically smaller and more clear, the 

 other larger and more granular. The latter undergo invagination or embolt, 

 that is to say they sink inwards, obliterating more or less completely the 

 blastocoele. The result is an invaginate, or embolic Gastrula, an ovate 

 or spherical body composed of a double layer of cells, an outer epiblast 

 ( = ectoderm), an inner hypoblast ( = endoderm), separated or not by a 

 space, the remnant of the blastocoele, and continuous at the blastopore 

 or Gastrula mouth. The central cavity into which the blastopore leads is 

 the archenteron 2 . 



Instances of typical or equal segmentation are met with in most 

 groups of multicellular animals, but the process is commonly modified 

 by the accumulation of food-yolk ; the blastocoele may be absent or only 

 slightly indicated, and transformation into a Gastrula is sometimes carried 

 out by an invagination, sometimes by a modified form of invagination 

 known as overgrowth or epibole, that is to say the epiblast grows round 

 the hypoblast or yolk. The food-yolk may accumulate at one extremity 

 of the oosperm, or in its centre ; to these two types the terms telolecithal 

 and centrolecithal are respectively applied. A telolecithal ovum may 

 segment completely but unequally, the hypoblastic cells being larger and 

 dividing more slowly, or its segmentation is partial and confined to a 

 disc at one pole, a large amount of yolk remaining unsegmented ; nuclei 

 appear in it, however, at the pole of segmentation, a certain amount of 

 protoplasm becomes segregated round them, and cells are thus added 

 to the blastoderm or segmented area. The extremes are connected by 

 transitional forms. Centrolecithal ova are confined to Arthropoda. The 

 central aggregation of the yolk may be present from the first, or take 

 place during segmentation. The yolk is in the latter case always massed 

 at the central ends of the blastomeres, which may or may not fuse, whilst 

 in the former case the furrows between the blastomeres are superficial, 

 i. e. do not penetrate to the centre of the oosperm. The central mass 

 of yolk thus left either does not segment at all or does so at a late 

 period, and the masses to which it gives origin are non-nucleate. The 

 blastomeres may be equal or unequal, their formation may be simul- 

 taneous or successive, and is very often preceded by a multiplication of 



1 The term ' morula ' is also applied to solid masses of cells produced by segmentation and not 

 yet definitively arranged in Gastrula-fashion. 



2 The blastocoele is sometimes open by one or more pores to the exterior, e. g. in the amplii- 

 blastula of Sycon (Sycandra}. It would have been better if the term blastopore had been restricted 

 by usage to such openings, and some such term as ' gastropore ' applied to the Gastrula mouth. 



