Cleavage, Blastulation and Gastrulation 



221 



such differentiation without cleavage is 

 blocked by interference with the cyanide- 

 sensitive system of oxidative metabolism, and 

 also by monoiodoacetic acid. 



The degree of differentiation obtained 

 without cleavage is limited, the most striking 

 feature being the formation of sm-face cilia. 

 An apical tuft is not found on svich embryos. 

 It appears clear that progressive differentia- 

 tion during normal development is de- 

 pendent on the progressive segregation of 

 different substances, and on their isolation 

 from each other by the cell boundaries be- 

 tween the cleavage blastomeres. Superim- 

 posed upon the independent differentiation 

 process under the "control" of such isolated 

 substances, there may be provision in some 

 forms for secondary effects to be brought 

 about across cell boundaries by diffusible 

 substances such as inductors or hormones, as 

 part of the dependent differentiation process. 

 Except perhaps in syncytial structures, cell 

 boundaries undoubtedly play a definite role 

 in normal development. Diffusion of sub- 

 stances across cell boundaries is a function 

 of the effective molecular size of the dif- 

 fusing substance and of the permeability of 

 the cell membranes involved. Obviously, 

 morphogenetic substances of relatively enor- 

 mous molecidar size are essentially nondif- 

 fusible. Such substances must be included 

 within the cells where their effects are to 

 become manifest, and cannot serve as in- 

 ductive agents acting across cell boundaries. 



FACTORS CONCERNED IN SHAPING OF 

 THE BLASTULA 



Since the process of segmentation results 

 in the production of many blastomeres, the 

 later arrangement of the numerous blasto- 

 meres with respect to each other is impor- 

 tant in relation both to the previous pattern 

 of cleavage and to the future fate of certain 

 areas. The arrangement of the cells of the 

 blastula differs markedly among species of 

 the various animal groups. If the blastomeres 

 become arranged as a solid ball of cells, a 

 morula or stereoblastula (the latter term 

 used by some authors) results. A flattened 

 morida is called a placula. If cleavage re- 

 sults in an arrangement of the blastomeres 

 around a cleavage cavity, this form is re- 

 ferred to as a blastula (or true blastula). 

 Various types of blastulae have been named, 

 including the coeloblastula, amphiblastula, 

 superficial blastula, and discoblastula. (Con- 

 sult Richards, '31, for definitions.) 



The factors concerned in the shaping of 



the blastula might be supposed to be inti- 

 mately tied up with the adhesion of the 

 cells to each other and with their relations 

 to the surrounding extraneous membranes of 

 the egg. These relations have been the ob- 

 ject of numerous investigations on various 

 echinoderm eggs, but relatively little has 

 been done on other forms. A typical fer- 

 tilized echinoid egg is surrounded by a rigid 

 fertilization membrane, which is separated 

 by the perivitelline space from the hyaline 

 plasma layer on the protoplasmic egg sur- 

 face. This hyaline plasma membrane is se- 

 creted by the egg cytoplasm, beginning a 

 few minutes after fertilization, and adheres 

 closely to the egg and to the daughter cells. 

 Herbst ('00) demonstrated that this layer 

 requires calcium for its stability, and it is 

 presumably a calcium proteinate. Radial 

 striae are sometimes visible in the layer. 

 Removal of the fertilization membrane may 

 be accomplished by shaking eggs vigorously 

 at the proper period, immediately after fer- 

 tilization, while the membrane is still soft; 

 this may be followed by treatment with 

 calcium-"free" (calcium-low) sea water to 

 dissolve the hyaline plasma layer (Herbst, 

 1892, 1893). Under such conditions, fine 

 anchoring strands were seen by Herbst 

 extending from the blastomere surfaces into 

 the hyaline layer. Dan ('52) considers these 

 strands identical with the radial striae. 



Gray ('24) has shown that the hyaline 

 plasma layer is a factor limiting cell and 

 spindle elongation during cleavage. It is 

 not a causative factor in cleavage (i.e., it 

 is not a surface-constricting layer), since 

 cell division can proceed after its total or 

 almost total removal in calcium-low sea 

 water. 



Moore ('45 and earlier papers) has dis- 

 cussed the effects on blastulation of re- 

 moval of the fertilization and hyaline 

 plasma layers of the fertilized echinoderm 

 egg. Moore obtained development without 

 "membrane formation" by treating unfer- 

 tilized eggs for a short time with a solution 

 of nonelectrolyte (urea or sucrose) isosmotic 

 with sea water, at a pH of about 8. The eggs 

 are then returned to sea water to be fer- 

 tilized. According to Moore, if an embryo 

 devoid of the hyaline plasma layer is al- 

 lowed to develop to a stage corresponding 

 to a blastula, the blastomeres form a flat 

 plate or sheet. The plate may have its 

 edges turned up. Motomura ('41), however, 

 washed embryos repeatedly and thoroughly 

 in a calcium-"free" medium, and found that 

 the blastomeres simply pile up, never form- 



