44 



General Part. 



1 a r g e^ and tte segmentation cavity is wanting, or is very 

 small. In sucli cases the gastrula appears to be formed in a manner 

 different from the foregoing : the epiblast cells, which originally form a 

 cap upon the hypoblast, gradually grow round it, and the hypoblast 

 itself often encloses no archenteron, but forms a compact mass. 



In the first and second types of gasti-ula formation described above, the 

 blastula grows directly into the gastrula by a modification of some cells to form 



the hypoblast (Fig. 29, 4, s). In the blastula, 

 the outer end of these cells is the wider, 

 but they gradually alter in shape until the 

 inner ends ai'e wider than the others, which 

 results simply from a different ari-angement 

 of then- protoplasm (protoplasmic move- 

 ments). This change in the shape of the 

 cells must necessarily lead to a flattening of 

 one side of the blastula and then to its 

 invagination. Simultaneously an alteration 

 occurs in the epiblast cells : these become 

 shorter and wider, so that they can extend 

 over a large ai-ea. Whilst the invagination 

 remains, the original relations of hypoblast 

 and epiblast are i-etained. The epibolic 

 gastrula (gastrula by overgi-owth) described 

 under (3), is probably formed in the same way. 

 There is no need to assume that the epiblast cells get loose from the hypoblast 

 and grow over it. It seems that the wide outer ends of the hypoblast cells 

 (Pig. 33, 1) gradually become narrower, and the inner ends broader, whilst the 

 epiblast cells at the same time spread out, so that, by the same process as in 

 the invagination of the typical gasti-ula, they gradually enclose the hypoblast, 

 without its being possible to speak of an active migration of cells, and without 

 the contiguous surfaces of the epiblast and hypoblast cells being altered. 



Fig. 32. Gastrula of a maiine 

 ■Gastropod (Natica). formed in the 

 same way as the gastrula in Pig. 

 31, from which it differs in the 

 possession of an archenteron. — 

 After Bobretzsky. 



Fig. 33. Diagrammatic figure of the formation of the epibolic gastrula (See Text). 

 1 youngest, 3 oldest stage, c hypoblast. The letters a and 6 indicate the same points in 

 all three figures — Orig. 



4. In many lower Vertebrata (Cyclostomi, Sturgeon, some other 

 Pisces, Amphibia) the blastula wall does not consist of a single layer 

 as in the cases mentioned above, but is many cells thick (Fig. 34, l) ; 

 the cells are larger on one side of the sphere than on the other, and 

 contain much yolk. This part of the blastula is invaginated, and a 

 gastrula is formed as before (Fig. 34, 4) ; but part of the hypoblast 

 is very thick, so that it stands up into the archenteron as a large boss. 



