XVII 



PEOTOCHOEDATA 



615 



of the neural plate. The four posterior are the rudiment of 

 the notochord (Fig. 444, A). The hlue endoderm cells divide chiefly 

 into anterior and posterior cells, but in front the lateral endoderm 

 ceU on either side divides into an inner and an outer daughter cell. 

 The outer cell on either side contains less yolk than the inner, 

 and is the so-called anterior mesenchyme cell, which is the rudi- 

 ment of the most anterior part of the mesoderm. 



Of the six yellow cells, the two outer ones on either side divide 

 into dorsal and ventral components. The dorsal component con- 

 tains less yellow substance than the ventral, and appears of a paler 

 colour and gives rise to mesenchyme. The inner cell on each side 

 divides into anterior and posterior 'daughters ; the anterior daughters 

 with the ventral daughters of the outer cells form the rudiment of 



Fig. 444. — Illustrating the gastrulation of the egg of Gj/w^Aia partite. (After Conklin.) 



[The coloured cytoplasms are indicated as in the two previous figures.] 



Both figures represent eggs viewed from the vegetative pole, i.e. from above and behind. 



A, stage in which the semicircular dorsal lip of the blastopore is just beginning to form. B, stage in 

 which the invagination of the endoderm is well advanced, a.mcli, anterior mesenchyme cell ; ch, noto- 

 chordal cells ; mchf mesenchyme cells ; n.p, neural plate cells ; p.mcli, posterior mesenchyme cell. The 

 dotted cells which are not mesenchyme are mesodermal cells. 



the longitudinal muscles of the tail of the larva, while the posterior 

 daughters of these inner cells form mesenchyme and constitute the 

 posterior mesenchyme cell on each side. The divisions of the cells 

 of the vegetative hemisphere take place first, and these cells pass into 

 a statei^bf rest, during which they alter their form and become columnar, 

 whilst ,the cells of the animal hemisphere are dividing. These latter, 

 on the contrary, now cease to he columnar and become flat (Fig. 445). 



If we make a register of the whole sixty-four cells, we find : 

 animal hemisphere, twenty-six ordinary ectoderm cells, six neural 

 cells ; vegetative hemisphere, four neural cells, four notochordal cells, 

 two anterior mesenchyme cells, four mesenchyme cells, two posterior 

 mesenchyme cells, six muscle cells, and ten endoderm cells. 



After this cleavage we can no longer speak of a general period 

 of cleavage affecting all the cells of the egg, because one set of cells 



