Cleavage, Blastulation and Gastrulation 



223 



blastulae are smaller. The intrahyaline liq- 

 uid which surrounds the egg at the first 

 cleavage is, therefore, the same material as 

 that which later fills the blastocoele. 



In support of these ideas of the mechanism 

 of blastulation, Dan cites one of the visible 

 events that occtus as the primary mesen- 

 chyme cells wander into the blastocoele. In 

 Acanthocidaris, he observed that the hyaline 

 layer is lifted off in blisters where the 

 mesenchyme cells escaped from their sur- 

 face position to wander inside. These little 

 blisters in the hyaline layer are soon pinched 

 at their bases and become bag-shaped. This 

 presumably occurs because when a mesen- 

 chyme cell slips away, the adjacent cells 

 come together to close the vacant space, 

 causing a slackening of the hyaline layer 

 above. 



The role of the various components in the 

 process of blastulation may be summarized 

 as follows: Immediately following fertiliza- 

 tion an echinoderm egg secretes the hyaline 

 plasma substance which gelates on contact 

 with the calcivim ions present in sea water. 

 After this, the egg simply adheres by proto- 

 plasmic processes to the "frame" which it 

 has made for itself. At intervals the hyalo- 

 plasmic envelope is distended by the periodic 

 secretion of some colloidal substance — prob- 

 ably a protein. This substance takes in water 

 and gradually accumulates as the blasto- 

 coelic fluid. The sitviation may perhaps be 

 considered analogous to a condition where 

 many people are inside a rubber ball pro- 

 vided with numerous straps hanging from 

 the inner wall. If the ball is inflated, the 

 "straphangers" form a layer adjacent to 

 the wall, leaving the central portion vacant. 

 This is the blastula. Sooner or later, some 

 of the "straphangers" release their straps 

 and fall (mesenchyme cells); thereupon, 

 the people around them come closer to- 

 gether to fill the gaps. The embrj^o is 

 thus led passively to its full size as a 

 blastula by the hyaline plasma layer. Up 

 to this stage, the size of the blastula can 

 be controlled or reversibly changed by 

 regulating the degree of swelling of the 

 hyaloplasmic envelope. 



Lewis ('49) has described "superficial gel 

 layers" on the surfaces of embryos. They 

 are reported to function in wound healing 

 and in cell movements. These gel layers are, 

 according to Lewis, always in a state of 

 contractile tension. The relation of gel layers 

 with this property to the "supracellular sur- 

 face coat" (Holtfreter, '43a) is not clear 

 (Lewis, '49). 



GASTRULATION 



The continuous transition that character- 

 izes development leads to the transforma- 

 tion of the monoblastic blastula into the 

 diploblastic gastrula. If a blastocoele was 

 present, a new cavity, the gastrocoele (primi- 

 tive gut or archenteron) characteristically 

 appears. Gastrulae forming from solid moru- 

 lae may have their inner (entodermal) layer 

 compactly solid during the early stages of 

 gastrulation (e.g., Nereis), the proctodeum 

 forming subsequently. The two primary 

 germ layers formed during gastrulation are 

 the entoderm (inner or hypoblast) and the 

 ectoderm (outer or epiblast). During or im- 

 mediately following gastrulation (and ac- 

 tually as part of this process), the third 

 primary germ layer, the mesoderm (or 

 chorda-mesoderm) arises from one or the 

 other of the original two layers and between 

 them. The coelom, where it exists, develops 

 within the mesoderm. The layers of the gas- 

 trula in many forms do not necessarily rep- 

 resent completely segregated germ layers. In 

 the starfish, the inner layer has been called 

 the mesentoderm because it gives rise to 

 mesoderm as well as entoderm. In the Am- 

 phibia and Amphioxus it gives rise to ento- 

 derm, mesoderm and notochord. The outer 

 layer gives rise to ectoderm in the starfish 

 and the anamniotes, and to ectoderm, meso- 

 derm, and notochord in the amniotes. In 

 this latter case the outer layer is called the 

 mesectoderm. 



The two primary layers of the gastrula 

 may arise in different species as the result 

 of very different processes. The best known, 

 or typical, form of the gastrula results from 

 invagination of the cells of the vegetal re- 

 gion of the blastula. The primary cavity of 

 the blastula (blastocoele) is to a greater or 

 lesser extent decreased in volume as the 

 archenteron appears. The blastocoele may 

 later be almost completely obliterated, or at 

 least considerably reduced by the continued 

 movement of the inner layer toward the 

 outer, and the opening (blastopore) of the 

 secondary cavity is reduced and sometimes 

 closed by the movements of intvirning cells 

 converging and narrowing this aperture. 

 Gastrulation may take place in certain spe- 

 cies by other processes, including the split- 

 ting off of a lower layer from an upper 

 (delamination), as has been described for 

 the development of birds. Another type of 

 gastrulation is by epiboly (overgrowth) of 

 mesentodermal cells by spreading and rap- 

 idly growing ectodermal cells of a morula. 



