164 EMBRYOLOGY. 



Formation of the Lamellae of the Blastoderm. 



From a comparative study of sections of the egg of the trout 

 at successive stages, from that at which the blastoderm begins 

 to form a cover over the saucer-shaped depression, consisting 

 of a middle thinner, and a peripheral thicker part (marginal 

 swelling Eandwuht}. to that at which it has already grown 

 round a quarter of the yolk and exhibits the first trace of the 

 formation of an embyro, the following facts may be made out: 

 The large elements found on the floor of the cavity gradually 

 tend towards the periphery of the blastoderm, where the}- form 

 the peripheral thickening, or marginal swelling already men- 

 tioned (see fig. 168). As this occurs the central part of the 

 blastoderm by degrees becomes so thin, that it consists at 

 length of only two layers of cells, an upper lamella of flattened 

 elements, and a lower containing loosely arranged spherical 

 elements (in single or, here and there, in double series). 

 These two layers are continuous with the marginal swelling, 

 the upper layer of which also consists of flattened elements, 

 the lower of one or two strata of more or less cylindrical cells. 

 In the marginal swelling two other strata exist underneath 

 these layers, each of which consists of large spherical elements, 

 and is at least two cells deep. We have therefore in the mar- 

 ginal swelling, by the thickening of which the rudiment of the 

 embyro is formed, four laj r ers, the upper or corneal layer 

 (Hornblatt) ; a second, or, as it may be termed, nervous 

 stratum, because out of it is formed the central nervous sys- 

 tem ; a third or motor-germinative ; and a fourth, or epithelial 

 glandular layer (Darmdrusenblatt}. Of these four layers the 

 two lower must be attributed to the formative elements which 

 come from the floor of the cavity. 



To the conditions just described those found in the batra- 

 chian egg are analogous. The mode in which, during the 

 formation of the cleavage-cavity, formative elements spread 

 from its floor over the under surface of the dome, adding a 

 third stratum to the two of which it already consists, has been 

 already described. This third layer then splits into two, 

 whilst the visceral cavity is growing upwards into the dome. 

 At a point which corresponds to the central part of the cavity 

 the cortex becomes thicker : this thickening, which is formed 

 at the cost of the second layer, is the rudiment of the central 

 nervous system of the embyro. We find the same four layers 

 in the egg of Batrachia the corneal, the nervous, the motor- 

 germinative, and the epithelial glandular (Darmdrusenblatt) : 

 the last two of which, as in the ovum of the trout, are derived 

 from the formative elements of the floor of the cleavage-cavity 

 (see fig. 173). 



Cleavage Cavity of the Chick. For the study of the 

 cleavage process and formation of the cleavage-cavity, in the 



