OF ARTICULATA. 299 



The first distinct trace of organization is to be found at the sur- 

 face, where a well-marked layer of large cells (fig. 198. c) encom- 

 passes nearly the whole circuit of the body. At the point where 

 this layer is deficient (ac l to ac 2 ) the yolk granules (y) extend to 

 the surface, whilst that part of the body over which the layer of 

 cells is spread appears lighter and less opaque (d). This layer 

 (c) might be compared to a ball-cover split open on one side, 

 and then we should have the split trending across the back of 

 the embryo, and one half of the cover representing the head, and 

 the other half the posterior region of the body. You may see 

 from this that the embryo is, as it were, doubled upon itself, and, 

 I will add, in anticipation of what will appear presently, in such 

 a way that its back lies next the centre of the ball, i. e., it is 

 doubled upon itself backwards. Which end is the head does 

 not become apparent until the next ^ 

 phase. At the latter period (fig. 199) 

 we find the layer of large cells, the ger- 

 minal layer, and the region which it " K^H^BHEESBSSBS^ 

 covers, seemingly narrowed, or con- 

 tracted upon itself (ab) along about two y 1 

 thirds of its length, but at one end (k) 

 rather more widely extended than in the 

 previous phase, so that, on the whole, 



when seen in front, it resembles a clumsy sort of Y. We have 

 now, at the broader end of this figure, the first indication of the 

 position of the head, (&,) but as yet no intimation anywhere as 

 to whether the embryo is an Articulate or Molluscan, or any one 

 of the five types of animals. 



In the next stage, (fig. 200,) however, the true character of 

 the animal, as far as its general typical relations are concerned, 

 begins to develop, under the form of a series of indentations 

 (i, i 1 ) which divide the anterior' half of the body from point to 

 point along the front or abdominal side, into successive joints 



Fig. 199. The same as fig. 198, at a more advanced stage. 220 diaui. k, 

 the head in profile ; k 1 , the right side of k in the distance ; A; 2 , the left side of A", 

 next the observer ; ab, the abdominal region ; tl, the posterior end of ab ; y, y 1 , 

 the yolk. From Zaddach. 



